As the deployment of photovoltaic (PV) power stations on slopes becomes increasingly prevalent, the seismic safety of PV slopes along expressways has garnered significant attention. However, existing designs frequently overlook the effects of far-field long-period (FFLP) ground motions on the degradation of soil plastic modulus and cumulative damage. Taking a real-world expressway PV slope as a case study, this paper systematically investigates the dynamic response of PV slopes under ordinary ground motions (OGM), far-field non-harmonic ground motions (FNHM), and far-field harmonic ground motions (FHM). The study utilizes the P2PSand generalized bounding surface plasticity constitutive model and employs a pile-soil coupled implicit numerical method. The constitutive model was calibrated via direct simple shear (DSS) tests to accurately capture the cyclic nonlinear behavior of the soil. The research focuses on quantifying the differential impacts of varying ground motion characteristics on soil plastic modulus, effective stress, shear strain increments, and displacement deformation. The results indicate that the long duration and low-frequency characteristics of FFLP lead to significant cumulative damage; even at a lower peak acceleration (0.10 g), the induced slope displacement exceeds that caused by high-amplitude (0.40 g) OGM. In particular, FHM, characterized by quasi-harmonic features, significantly accelerates the degradation of plastic modulus and cumulative damage in shallow soil layers due to its frequent periodic characteristics and continuous narrow-band energy input, resulting in excessive permanent displacement of both the slope and the PV structure.