Millimeter wave (mmW) multiple-input multiple-output (MIMO) radar is state-of-the-art technique for security imaging. However, the exploitation of large number of transmit/receive channels leads to much higher cost and complexity for the MIMO radar than that of the single-channel radars. Aiming for developing the low-cost and low-complexity mmW MIMO security imaging radar, dual-code-division-multiplex (dual-CDM) technique has been proposed in this paper. It is worth noting that the proposed method can realize the similar imaging performance with the traditional multi-channel MIMO radar by a single transmit /receive channel, which is of great significance for the development and application in the area of security imaging. Moreover, the de-multiplex code, which can attain the maximum code diversity gain against the interference, is designed based on mismatched filter theory. Finally, both the numerical and the experimental results are provided to verify the correctness and the validity of the proposed methods, which shows that quite a satisfactory imaging performance is achieved.