The invention discloses a system reserve capacity decision-making method with consideration of a demand response. On the basis of demand elasticity estimation, a user outage cost value of a user is estimated; and a system reserve cost value includes a capacity cost value and a real-time transfer power cost value and a system reserve cost value is estimated according to a capacity cost calculation model and a real-time transfer power cost calculation model. With a minimum sum of a user outage cost value and a system reserve cost value as an objective, a system optimal reserve capacity value is calculated by considering constraint conditions like system power balance, minimum reserve capacity limit, generator set output, upper and lower limits of spinning reserve, and system static safety. According to the system reserve capacity decision-making method, demand-side resources are introduced into standby resources, so that the reserve capacity of the power generation side can be reduced and the system equipment utilization rate can be increased.