The problem of optimal standby capacity and its valuation in Power Pool is very important and complicated. On the basis of researching literatures concerned, this paper constructs an optimal standby capacity model and researches the problem of pricing optimal standby capacity and determining power price on the user-side. The result shows that the optimal standby capacity at some period t is completely determined by the probability distribution density function of power load demand at this period and surplus power loss and insufficient power loss, and the price of optimal standby capacity is determined by real power price of entering power grid and the power price of forecasting load demand, and the expected power price on the user-side consists of the settlement price with grid loss on generation-side and reasonable gains and expected additional loss.