The traditional methods of total supply capability（TSC）calculation，including the TSC model and analytical method，have ignored the impact of the voltage constraints and losses on TSC.This paper analyses the impact of voltage constraints and losses on TSC based on power flow.Firstly，an N-1 security verification method based on power flow is given，which has accurately considered the losses，the voltage constraints and voltage-regulating measures.Secondly，this paper compares and verifies the results of TSC in 5 scenarios，including whether considering voltage constraints or not and considering the voltage-regulating measures separately or at the same time.The results show that the value of TSC calculated by the existing methods is rather great.The impact of losses on TSC cannot be ignored as the power supply path will be longer after load transfer when N-1 occurs，therefore the terminal voltage may become too low and the losses will be larger than normal.After considering the voltage-regulating measures，the normal voltage constraints can be ignored.The work of this paper has laid a solid foundation for more accurate TSC calculation.