Abstract: [Objective] Landslide disasters represent complex geomorphological hazards occurring at the Earth's surface under the combined influence of internal and external geological dynamics. Although tectonic stress fields significantly affect the evolution and spatial distribution of landslides, quantitative research regarding the specific impact of tectonic stress on landslide development remains insufficient. This study aims to quantify the influencing effects of tectonic stress fields on landslide distribution across different geomorphological zones in the upper and middle reaches of the Dadu River area. Dadu River area lies in the transition zone between the eastern Tibetan Plateau and the Sichuan Basin—a region known for intense tectonic activity, rugged topography, and frequent landslides. [Methods] Focusing on four distinct zones (S1-S4) with concentrated landslide distributions, a newly developed in-situ stress depth–intensity model for the southeastern Tibetan Plateau was introduced. The horizontal tectonic stress fields and topography-induced stress fields (generated by gravity) were quantitatively calculated at grid points within the landslide boundaries of each zone. These calculated stress fields were subsequently integrated with slope geometric characteristics (slope length, gradient, and aspect), landslide attributes (area, frequency, and sliding direction), and regional geological-geomorphological backgrounds to analyze the differentiated mechanisms of tectonic stress on landslide evolution. [Results] Significant variations in the influence of tectonic stress on landslide development were observed across the four zones: (1) Upstream subdivision (S1): The proportion of tectonic stress is the lowest across the entire Dadu River area (approximately 50%). The landslide slope length probability (R_L) increases monotonically with slope length (L), aligning closely with the theoretical curve dominated by topographic stress, which indicates that landslide development in S1 is primarily governed by gravitational topographic stress. (2) Midstream subdivisions (S2, S3):Situated at the intersection of multiple fault zones with frequent seismic activity, the proportion of tectonic stress increases markedly to approximately 63%. The landslide slope length probability (R_L) exhibits a uniform and stable distribution within the 1 km to 3.5 km range. This pattern indicates a high proportion of earthquake-induced landslides, where instantaneous stress release from seismic events triggers slope instability that correlates weakly with slope length.; (3) Tributary (Liusha River) subdivision (S4): Although the calculated tectonic stress proportion approaches 80% with a peak landslide probability occurring at shorter slope lengths (approximately 1 km~2 km), wide exposures of low-strength mudstones and sandstones cause the calculated tectonic stress to be significantly overestimated compared to actual conditions; landslide development in S4 is ultimately influenced primarily by lithology, leading to the concentrated clustering of short-length and low-gradient landslides. [Conclusion] In conclusion, landslide development in the upstream subdivision (S1) is primarily influenced by topography-induced stress, while the midstream subdivisions (S2, S3) are significantly affected by tectonic stress. The instantaneous release of tectonic stress caused by earthquakes breaks the slope stress equilibrium, and the instability process has a weak correlation with slope length. The tributary subdivision (S4) is mainly influenced by weak lithology rather than tectonic stress. Overall, tectonic stress, as a component of the initial stress field, plays a crucial role in slope stress equilibrium. Under the continuous action of external forces such as weathering, rainfall, and earthquakes, the tectonic stress is weakened due to the reduction of the elastic modulus, leading to mechanical imbalance and subsequently influencing landslide development. [Significance] The differing characteristics of tectonic stress factors influencing landslide development across various subdivisions of the Dadu River basin offer new insights into understanding the spatial distribution patterns and mechanisms of landslides. Meanwhile, considering the impact of rock mass mechanical property degradation over time on the initial stress field may be a feasible direction to connect long-term geological evolution with short-term landslide triggering processes. This study employs a quantitative mechanical approach to reveal the key factors influencing landslide development in different sections of the Dadu River area, offering new insights into the role of tectonic stress in landslide spatial distribution patterns.