Abstract: As to the influence of the operation status of sending system on transmission power, the operation characteristics of the HVDC rectifier connecting to various strength AC systems are studied. By combining with operation status of the inverter that connect to various strength AC systems, it is pointed out that the size relation of rectifier and inverter SCR (short circuit ratio)can affect maximum transmission power of HVDC system. Firstly, according to power curves of rectifier and inverter, the quantitative relationships between maximum transmission power and SCR are achieved respectively. Through comparing rectifier and inverter SCR under the same maximum transmission power condition, the turning point of SCR (TSCR)is defined as that obtained when maximum transmission powers of two terminal systems are equal. Finally, it is pointed out that the SCR of both inverter and rectifier sides would affect the transmission capacity. When there is certain relationship between rectifier and inverter SCR with TSCR, the transmission capacity of HVDC system is limited by one terminal side. Simulation results in PSCAD show the correctness of phenomenon founding.