For safety transmission, a security method in the physical layer is proposed based on generalized spatial modulation technique. The transmitter pretreats the signals according to the state information of the legal channel so that phases of all signals sent by different antennas are identical at the legal receiver. At the same time, different additional phase shift is imposed when different combination of the activated antennas is used, so the performance of the legitimate user is improved. However, phases of signals sent by different antennas are random at the eavesdropper, thus its performance is significantly lower than that of the legitimate user so that the information conveyed by the antenna index is protected. Artificial noise pointed to the eavesdropper is sent simultaneously at the transmitter to safeguard the information conveyed by amplitude and phase modulation symbols. Then the secrecy capacity, error performance and power assignment between signal and artificial noise are analyzed. The simulation results show that the legitimate receiver's error performance is superior to that of the eavesdropper, and a considerable secrecy capacity can be obtained.