The invention relate to a hybrid planning method for medium-and-long term power transmission network expansion plan in an electricity market environment. The method includes a step 1 of setting a target function of a hybrid planning model; a step 2 of setting basic constraint conditions for power grid operation; a step 3 of detecting a power grid by utilizing an N-1 mandatory constraint condition and an N-2 flexible constraint condition; a step 4 of judging whether specific reliability constraint conditions are needed for enhancing the power grid so as to enable the power grid to meet the N-1 mandatory constraint condition and the N-2 flexible constraint condition; a step 5 of searching an optimal planning scheme for simulation calculation. The technical scheme provided by the invention can provide new evaluation indexes, new target functions and more reasonable and comprehensive reliability and economy constraint conditions for medium-and-long term power transmission network expansion plan. The method has characteristics of simpleness, practicability, comprehensiveness and high operability.