Abstract: With a large number of synchronous generators being replaced by infeed direct current and new energy generation, the system inertia level has decreased, frequency modulation resources have decreased, and the problem of frequency stability has become increasingly prominent. LCC-HVDC(line-commutated converter based high-voltage direct-current) is widely used. It is an effective means to improve frequency stability by using power quick adjustment ability of LCC-HVDC to provide frequency support for the system. In this paper, based on the basic framework of power step frequency control, the output timing optimization model of LCC-HVDC is established when the lowest frequency occurs at the highest as the target after power disturbance considering the output limit of LCC. Based on the sequential quadratic programming method, the optimization problem is solved to determine the best additional power and support time of LCC. Simulations based on MATLAB/Simulink verify the effectiveness of the proposed frequency support control strategy.