In the electricity market environment， there exist risks in determining the spinning reserve capacity and its allocation. In order to analyze the risks of spinning reserve scheme from the prospective of power generating system， a stochastic variable called spinning reserve's gain or loss is defined. The probability distribution of spinning reserve's gain or loss is studied and the standard semi-variance is used to measure the risk of a spinning reserve scheme. The utility function and utility value are used to reflect different decision makers' satisfaction levels to the spinning reserve's gain or loss figure（conservative， risky， or neutral）. A utility expectation decision-making model is proposed. It not only reflects the objective risk of a spinning reserve scheme， but also the decision-maker's attitude to risks， which makes the model more practical. A case study shows the validity of the presented model.