Highway photovoltaic slopes have been treated as an emerging application. To investigate their stability and improve the computational efficiency of three-dimensional models, a study was conducted based on the associated flow rule and a three-dimensional photovoltaic-slope model was developed in FLAC^3D. An improved strength reduction method (SRM) was proposed, and the integration and optimization of stress-field application, timestep upper limits, and strength-reduction factors were implemented using Python to substantially accelerate the calculation of the safety factor (FS). The effectiveness of the proposed method was verified by means of a field photovoltaic-slope case. On this basis, the sensitivity of FS to single-factor influences (cohesion, internal friction angle, slope ratio, photovoltaic pile length, and photovoltaic structure position) and to multi-factor coupling effects was analyzed. The results indicate that the improved SRM can markedly enhance computational efficiency while preserving calculation accuracy, and that the installation of photovoltaic power systems on slopes exerts limited influence on slope stability, thereby demonstrating the feasibility of large-scale deployment of photovoltaic slopes.