Retaining certain percentage power reserve is one of the most important measures to ensure the safety and reliability of the power system. With the minimization of transmission investment as objective， a new transmission network planning model considering the power reserve is proposed. Two types of constraints are taken into account: one is that the power reserve can be used when any generator is outaged; the other is that the power reserve can be transferred to maintain the safety of the system in the case of uncertain load increasing. An improved greedy randomized adaptive search algorithm is used to solve the model quickly and effectively. Simulation results on the 6-bus system and 46-bus system validate the new method.