In the new energy environment, the uncertainty of load demand and new energy power generation puts forward higher requirements for reserve capacity, and new energy and reserve replacement receive increasing attention. Based on the relationship between expected energy not supplied（EENS）and reserve capacity, total reserve capacity of the power system is determined and capacity is allocated among various types of generating units in a preferential way. Based on EENS and operation cost, the evaluation index of comprehensive substitution benefit of photovoltaic power generation system replacing thermal power units is constructed. The value of comprehensive substitution benefit evaluation index is minimized as the objective function, and the optimization model of backup capacity of power system considering the photovoltaic power generation system replacing thermal power units in backup operation was constructed. The piecewise linear theory is used to linearize the objective function with quadratic characteristics, and the branch-and-bound algorithm is used to solve the nonlinear integer programming problem. Taking the IEEE- 30 system as an example, the calculation results show that the photovoltaic power generation system replaces the thermal power unit in the standby operation, which can reduce the comprehensive substitution benefit and reserve capacity.