In the multi-input multi-output (MIMO) wireless communication systems, spatial modulation (SM) has wide application prospects as a novel modulation technology. In this paper, a new secrecy enhancement scheme for the SM system is proposed, in which the number of transmit antennas $N_{a}$ is not a power of two. The transmitter first chooses $N_{t}$, which is a power of two, antennas from $N_{a}$ transmit antennas for SM. Then, the transmitter transmits, in addition to the information symbols, an artificial noise (AN) through the active antenna. At the same time, another one of the remaining $N_{a}-N_{t}$ antennas is activated randomly by the transmitter to transmit another AN. The ANs are designed by exploiting the channel state information (CSI) of the legitimate channel, and then the ANs can only be cancelled at the legitimate receiver while the passive eavesdropper will suffer the interference of them. Furthermore, the security performance of the proposed scheme is also analyzed. Numerical results verify that our proposed scheme is able to achieve a satisfactory performance.