Abstract: [Objective] Accurate characterization of formation fracture complexity is of great significance for evaluating lost circulation risks, hydraulic fracturing stimulation effectiveness, and various operational stages throughout the oil and gas drilling and production lifecycle. To address the limitation that existing fracture characteristic parameters cannot comprehensively represent fracture complexity, this study proposes a method for establishing a 3D fracture complexity index based on the Well-Seismic integration concept. Using an Oilfield in the Bohai Sea, China, as a case study, this method enables refined characterization of formation fracture complexity. [Methods] Considering the influence of both fracture aperture and fracture intensity on fracture complexity, a 1D fracture complexity index model based on expert decision-making and well-logging data is established using a combined subjective and objective weighting approach that integrates the Analytic Hierarchy Process and the Entropy Weight Method. Furthermore, the fracture development degree attribute volume derived from seismic inversion is used as a constraint condition for Kriging interpolation to construct the 3D fracture complexity index attribute volume. [Results] Using this method, the 1D fracture complexity index profiles of drilled wells are derived. A comparison with borehole image logs shows that a higher fracture complexity index corresponds to a greater number of fractures or larger fracture widths at the corresponding depth, confirming the feasibility of this method for characterizing fracture complexity. In the 3D fracture complexity index model, the attribute volume profile along the drilled well is extracted and compared with formation lithology and dual laterolog profiles. The results indicate that a higher fracture complexity index corresponds to a greater difference in dual laterolog responses. Furthermore, the lithology of this well interval is identified as buried-hill granitic gneiss, further validating the reliability of this method. [Conclusion] A fracture complexity index was constructed using a combined weighting approach that integrates the subjective Analytic Hierarchy Process and the objective Entropy Weight Method. This index not only reflects the extent of fracture distribution but also captures the internal structural heterogeneity of fractures. It addresses the issue of low prediction accuracy in existing lost circulation risk prediction methods, which rely solely on fracture intensity as the fracture characteristic parameter. Moreover, it compensates for the limitations of current hydraulic fracturing stimulation evaluation methods, which assess compressibility based on a single factor, such as fracture intensity or fracture aperture. [Significance] The research results can provide theoretical references and engineering guidance for lost circulation risk prediction and compressibility assessment.