The hydropower which sends out corridors in the Sichuan west provincial grid consists of multi-circuit 500 kV AC transmission lines. The AC system of the Sichuan grid bears the low frequency operational risk when serious faults occur on large capacity AC system transmission corridors. To promote Sichuan grid transient stability and help to recover AC system frequency, a study is performed on the AC/DC system coordinated control strategy to improve Sichuan grid operation performance. The coordinated control strategy includes: 4 circuit ±800 kV UHVDC to obtain coordinated control with the AC system using a DC power modulation function. This provides the desired outcome of coordinated control, optional modes of coordinated control for different fault locations (distributed/centralized control modes), an analysis of the influence on AC grid heat stability security and grid operation economical efficiency when UHVDC DC modulation power transferred to the AC system. The AC/DC system coordinated control strategy introduced by this paper can help to analyze the AC/DC system operating condition of a AC/DC co-existent grid and be regarded as the technological measure reference for grid security and economical operation enhancement. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGSCJY00GHJS1800026).