[Objective] The overall performance of prefabricated dams depends on the cementitious materials between precast blocks. However， current research on the impact of the deterioration of cementitious material performance on the overall performance of the prefabricated dam remains insufficient. [Method] Taking the No. 3 water-retaining dam section of the main dam at the Lushui hydro-junction as the research object， numerical simulations were conducted combining thin-layer elements and the finite element method. The impact of the bonding performance of cementitious materials between precast concrete blocks on the overall performance of the dam was quantitatively analyzed. Furthermore， the differences in the impact of performance deterioration in various types of joint cementitious materials on the prefabricated dam were investigated. [Results] （1） As the degree of deterioration of the cementitious joint materials increased， the displacement of the prefabricated dam gradually increased. When the deterioration of the cementitious material reached 90%， the displacement increased by approximately 21% compared to the baseline condition. （2） With the increasing deterioration of the cementitious material， the area of the maximum principal stress in the precast blocks gradually expanded， with the high-stress regions primarily concentrated at elevations of 30 m to 36 m on the downstream side. （3） As the deterioration worsened， the proportion of closed cementitious joints in the prefabricated dam gradually decreased， while the proportion of open joints increased， leading to an increase in dam displacement. （4） The deterioration of the horizontal cementitious joint materials had the most significant impact on the dam’s performance. When the deterioration of the horizontal joint cementitious material reached 80%， the thin-layer elements in a non-closed state were mainly located on the horizontal joints near the downstream side at elevations of 33 m to 38 m in the assembly area. [Conclusions] Thin-layer elements can effectively simulate the state changes of cementitious joints， demonstrating the feasibility of applying them to the performance calculation of prefabricated dams. The deterioration of cementitious materials leads to increased displacement and stress in the assembly area of the prefabricated dam， thereby threatening its safety performance.