The uncertainty of wind power has great influence to spare capacity. On this basis, a new calculation method of spare capacity considering wind power forecasting error is proposed. Firstly, considering the inflexible problem of Laplace distribution, an improved Laplace distribution is proposed. Secondly, a new method is proposed to calculate the spinning reserve capacity. First, taking minimum cost as objective function, the based operation point can be gotten by solving the optimal power flow. Then, it builds robust optimal power flow model by considering robust optimization and affine model of Automatic Generation Control (AGC) to solve the participation factors of AGC generators. The spare capacity can be solved combining equal probability conversion principle. From the simulation, the proposed forecasting error model is of high accuracy and the calculation satisfies the requirements of safety and economy. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900100).