In multi-controller Software Defined Network (SDN), since the existed switch migration strategies can not adapt to the change of switch traffic which only consider single migration factor, a Switch Migration Strategy based on Improved Gravitation Search Algorithm (IGS-SMS) was proposed. In the decision-making stage, the multi-objective decision based on fuzzy satisfaction was used to optimize the objectives by the competitive priority of membership. In the calculating phase, the objective function with the top priority was optimized by improved gravitational search algorithm. The simulation results show that the IGS-SMS achieves good load balancing of controllers while ensuring the index of transmission delay and switch redistribution. When local load was heavy in the experiment, Dynamic Switches Migration Algorithm (DSMA) and Progressive Auction based Switches Migration Mechanism (PASMM) couldn't alleviate overload. By contrast, IGS-SMS could alleviate overload, and load balancing degree was lower than DSMA and PASMM.