Moving Target Defense - False Data Injection Attack(FDIA) is a typical network attack, which can bypass the Bad Data Detection(BDD) and affect State Estimation(SE), the estimation results is vital for power system, thus posing a great threat to the security of power system. In this paper, a new defense scheme is proposed, which is based on flexible switching of spare lines. By switching on the spare lines of some working transmission lines flexibly, the transmission line parameters in the power system topology can be changed, so as to reduce the possibility of FDIA. The impact of switching spare lines on power system operation and FDIA by ergodic method is analyzed. An optimization algorithm is designed to find the least system generator cost for power grid operator and the least attack space for attackers, this algorithm is tested in the IEEE 5-bus system and IEEE 30-bus system, and the results show that the scheme has a good performance in resisting FDIA.