The invention discloses an AGC frequency modulation capacity calculating method suitable for the frequency modulation service market, and the method comprises the following steps: calculating the total adjustment amount needed in a time period at a time based on historical load data and carrying out the correction, and calculating the power generation output of new energy and carrying out the correction; giving consideration to the impact on the AGC capacity demand from the call wire exchange power and the deviation of the call wire exchange power; giving the impact on the AGC frequency modulation capacity demand from the classes of sets, calculating the adjustment amounts of different sets and the planned adjustment amounts of the sets, and calculating the adjustment capacity needed by AGC in each time period; obtaining the final needed AGC frequency modulation capacity in the time period; introducing a penalty factor p to correct the AGC capacity if one point violates a CPS2 rule or an A2 rule in any time period. The method can improve the economical performance of the auxiliary service market under the condition that the safety of a power system is guaranteed.