The promotion of demand response programs and the uncertainty caused by wind power integration create opportunity and challenge to transmission network expansion planning. To defer the transmission line investment with demand response, the transmission expansion planning model is proposed considering direct load control in generation re-dispatch for N-1 contingency. The extreme scenario method based on the convex hull vertices of history data is utilized to tackle with wind power uncertainty, which ensures the proposed model is a solvable mixed integer linear programming model. The modified IEEE 24-node system is tested with practical parameters. The function of demand response in relieving power flow overload and reducing transmission line investment is presented in the simulation results, which indicates demand response can improve the economy of power grid planning with the guarantee of system operation security. By adjusting the range of convex hull, the balance between economy and robustness of the proposed model can be reached. This work is supported by China Postdoctoral Scientific Fund (No. M2018642841), National Natural Science Foundation of China (No. 51707070), and Science and Technology Project of State Grid Hubei Electric Power Co., Ltd. “Technical Research on Large Hub Power Grid Active Distribution Network Coordination Planning”.