Abstract: [Objective] In order to reveal the synergy law of strike-slip faults under different normal stresses, this study systematically investigates the instability process of strike-slip faults through numerical simulation methods. [Methods] By using the numerical simulation method, based on the FLAC3D software and the frictional-hardening and frictional-softening model, a numerical model of strip-slip fault (elastic modulus 22.3 GPa, Poisson's ratio 0.25) is established. Six normal stress schemes (0.1~3.5 MPa) are set, and the loading rate is 0.5 mm/min for all. By comparing and analyzing the spatiotemporal evolution characteristics of the shear strain field of strike-slip faults under different normal stress conditions, the influence of normal stress on the evolution of the shear strain field and fault displacement is discussed. Based on the changes of the shear strain field and fault displacement, the degree of synergy is quantitatively determined. [Results] Under the same conditions, the normal strain perpendicular to the fault direction shows a decreasing trend with the increase of time steps; while the shear strain parallel to the fault direction has similar evolution patterns at different monitoring points but with different mean values. The mean value of shear strain at monitoring point 1 is negative, that at monitoring point 11 is positive, and the mean values at monitoring points 2 to 10 tend to zero, the monitoring points refer to the locations where data changes are obtained. In the sub-unstable stage, when the fault stress accumulates to the critical point, the shear strain in the weak areas within the system increases significantly first. The range of the concentrated shear strain area gradually expands and connects, eventually forming a continuous shear strain connected area. Normal stress is positively correlated with both coseismic displacement and shear strain, and the change in shear strain energy density is also positively correlated with stress. Normal stress has an important influence on the displacement in the sub-unstable stage. With the increase of normal stress, the synergy coefficient gradually decreases and the degree of synergy increases. In the sub-unstable stage, the synergy coefficient shows a significant downward trend. [Conclusion] Normal stress significantly affects the degree of coordination in the sub-instability stage of strike-slip faults by regulating the spatial distribution and release process of shear strain energy. The increase in normal stress leads to an increase in co-seismic displacement, an accumulation and enhancement of shear strain energy, and effectively improves the degree of fault coordination. The synergy coefficient can be used as a key indicator to quantify the degree of synergy before fault instability and has application value in identifying the sub-instability state of faults. [Significance] This study clarifies the positive correlation between normal stress and the degree of coordination of strike-slip faults, providing an important scientific basis for earthquake prediction and disaster prevention and mitigation.