Considering the impacts of rain-snow-ice weather on wind farm practical operation, this paper proposes a new short-term output model of wind farm. This model considers the ice accretion on wind turbines and transmission lines. The power output models of wind turbines with ice on blades are modified based on existing studies. The outage of facilities in wind farm is also considered in this model. The outage probability of wind turbines and transmission lines under bad conditions is estimated and a time varying model is established. Considering the wake effect, the states of wind turbines are sampled sequentially. Combined with the output level of wind turbines and the states of transmission lines, the power output of wind farm is calculated. Simulation result shows that the proposed model can reflect the short-term characteristics of wind farm output and can be used for short-term reliability assessment of power system.