Due to the increasing penetration level of tidal power in power system, the tidal turbines can provide less frequency support than conventional generators due to their small rotor mass. This makes the power system with low inertia and cause frequency problem. This paper presents a simulation model of a tidal power farm based on a MW-level variable speed tidal turbine with doubly-fed induction generator (DFIG) developed in the simulation tool of Matlab/Simulink. According to the reserve capacity required for primary frequency control, a de-loading control method is proposed in this paper to resolve the issue of primary frequency control via tidal power plant. Based on the analysis method of the frequency control characteristics of DFIGs, it is proposed by improved variable pitch control method. The control strategy, which is based on pitch control system of tidal turbine, is proposed in order to participate into primary frequency regulation of power system. Simulation results show that the proposed control strategy is effective means the tidal turbines with DFIG generators could providing frequency support for power system when they are working under the de-loading condition in this paper.