The invention provides an optimum day-ahead reserve capacity determination method. The method comprises the following steps: 1) analyzing uncertainty factors influencing system reserve capacity, and establishing uncertainty mathematic models of load, wind power and a traditional machine set considering demand response; 2) determining a power supply margin probability distribution function; and 3) determining the system day-ahead optimal reserve capacity according to load loss cost, wind cut punishment index and positive and negative reserve capacity cost. According to the method, comprehensively considering the uncertainty factors influencing the reserve capacity, association relationship of each influence factor and system standby demand is established, and thus the day-ahead reserve capacity is allowed to be determined more accurately and reasonably; the influence of user side demand response on the system reserve capacity optimization result is studied, and the effect of a demand response project on the aspect of reducing reserve capacity requirements is analyzed; and weighing is carried out between risk degree selection and reserve capacity, and the system day-ahead optimum positive and negative reserve capacity is determined.