A reliability model of wind farm is proposed. The output of wind farm is considered to obey a joint probability distribution of available capacity and available wind energy. In this study， the wind farm is equivalent to a multi-state conventional unit by dispersing the output into several derating output levels. The model allows the simulation of wind farms containing either identical or different wind turbine types. The model can be easily used in the power system reliability assessment utilizing analytical or simulation methods to simplify the reliability assessment of power system containing large-scale wind farm. Finally， a case study is carried out on IEEE-RTS 79 test system to verify the effectiveness of the model.This work is supported by State Grid Corporation of China.