Photovoltaic (PV) power generation systems usually operate at the maximum power point, and it is difficult for them to provide power support for the grid under emergency conditions. Constrained by construction and operating costs, large-scale applications of energy storage systems are still difficult to achieve. The reduction in the proportion of Synchronous Generators (SG) has increased the security risks of power grids with a high proportion of PV power generation. The power characteristics and stable operating conditions of a PV power generation system are analyzed, and a control principle based on PV power reserve is proposed to provide emergency power support for the grid. The relationship between the agreed power generation cost and the risk cost is analyzed. A two-stage risk reduction power generation optimization scheduling method is established with a start-up and shutdown plan of SG, reserve capacity and PV power reserve as decision variables. A power system risk reduction optimization scheduling method taking into account the PV power reserve is proposed. Finally, the correctness of the proposed method is validated on the IEEE RTS 24-node system. The case study demonstrates that the proposed method is the most economical when reducing the risk of power imbalance in the power grid and has good practicability. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGSW0000DKJS1900097).