Abstract: At present the determinate reserve capacity of primary frequency control (abbr. PFC) cannot satisfy the requirement of the sufficiency under large disturbance of power grid because the grid-connection of large scale new energy generating units replaces partial of conventional thermal power generating units. Along with the development of smart power grid, the demand side response resource represented by the electric water heater (abbr. EWH) is an excellent frequency regulation reserve, and by use of compensation mechanism the EWH can be stimulated to participate in the PFC. A dynamic response model of primary frequency regulation, in which the system inertia, the ramp rate of governor, the dead zone of frequency regulation and the load of EWH were taken into account, was constructed to form the constraint of the minimum PFC capacity for the unit. In the premise of considering the security constraint of PFC capacity the cost of ancillary services of PFC was brought into the operating cost of the system, and the objective function was constructed by the minimum operating cost and solved by Matlab-Yalmip-Cplex. Finally, the effectiveness and security of the constructed model were verified by the computing example based on the modified IEEE 30-bus standard test system. It is also verified that the EWH load can quickly respond in the early stage of frequency accident and effectively reduce the frequency deviation.