The seismic reliability of water supply networks refers to its capacity to satisfy water demands of customers in the presence of potential seismic events. Currently, the majority of seismic reliability assessments for water supply networks employ Monte Carlo simulations to generate numerous seismic damage samples for evaluation. However, this approach becomes highly demanding in terms of labor and time when assessing extensive and intricate networks. To address these challenges, the equivalent scenario method is proposed as a means to enhance computational efficiency. The proposed method involves the generation of posterior probabilities using seismic damage scenarios. Subsequently, these probabilities are employed to generate equivalent scenarios. Finally, the equivalent scenarios are utilized to determine both the node reliability index and the system reliability index. The simulation results obtained using the proposed algorithm are then compared with those obtained using the traditional Monte Carlo method.The results demonstrate the feasibility of generating equivalent scenarios based on posterior probabilities. Specifically, when evaluating the reliability of 100 equivalent scenarios at a seismic intensity of 8 degrees, a few nodes exhibit evaluation errors exceed 10%, while the evaluation errors for the remaining nodes at lower seismic intensities are below 5%. Additionally, the evaluation errors for small seismic intensities are all below 5%. Moreover, increasing the number of equivalent scenarios from 100 to 250 can further reduce the assessment error to less than 5%. Thus, the algorithm proposed in this paper ensures accurate results while enhancing computational efficiency.