Spatial modulation (SM) has attracted great attention of scholars as a novel multiple antenna technology. The optimal design for the constellation has been studied in recent years. However, since the structure of constellation diagram is predesigned, the existing algorithms are incapable of finding the optimal constellation diagram. For this reason, a new constellation design method is proposed under power constraint. The proposed method based on the minimized systems symbol error rate (SER) optimizes the constellation by expanding the feasible domain and by using the genetic algorithm. Furthermore, the influences of signal constellations on SER and of system parameters on the process of constellation design are analyzed. The simulation results indicate that constellations obtained by the proposed method are capable of offering a better SER performance compared with other constellations.