Wind power ramp event can bring serious impact on active power balance of power system, which may cause system frequency differential out-of-limit or load lost. A reserve demand analysis and preventive control model to deal with wind power ramp event is proposed to solve the above problems. Firstly, the probability distributions of wind power ramp event predication information in time series is converted to the probability distributions in single time sections based on sequence operation theory, and the reserve demand to deal with ramp event is analyzed based on it. And then, a preventive control model is proposed, the dispatching schedule is adjusted according to the newest predication information with it, and the adjustments include the curtailed amount of wind power, the output power and reserve capacity of conventional generators. The simulation results with IEEE RTS 24 system show that the proposed model can assess the operation risk of power system accurately and the operation risk can be controlled within acceptable range. This work is supported by National Natural Science Foundation of China (No. 51177092).