The invention discloses an alternating-current and direct-current power grid expansion planning method. The alternating-current and direct-current power grid expansion planning method comprises the following steps of establishing a VSC element model; setting a target function and a constraint condition of alternating current and direct current planning, and listing all possible planning conditions; according to the real-time running data in an alternating-current and direct-current hybrid power grid, performing direct current relaxation and droop bus power estimation on all the listed planningconditions; carrying out alternating current and direct current iterative calculation, and assuming a current converter to be predicted constant injection power, and taking an alternating-current power grid as constant injection power when the power flow of the direct-current power grid is calculated; finally judging whether the alternating-current and direct-current power grid power flow calculation is correct or not through the current converter data obtained by alternating-current power grid power flow calculation and direct-current power grid power flow calculation; and solving a numerical value of the objective function of each scheme for all possible plans, and selecting a proper planning scheme. According to the method, the numerical values such as power generation cost and the like can be accurately calculated, so that the planning result has higher reliability and practicability.