The harmonics of the return current may interfere with the signaling current along with the rails. In order to investigate the index of the compatibility with the signaling current, the theoretical investigation of the return current harmonics is preferable rather than expensive and time consuming experimental study. In this paper, we present the theoretical studies of the return current harmonics with the asynchronous type two-step PWM in the inverter-controlled DC electric railcar, aiming at contributing future work to improve the compatibility with the signaling current. We analyze the return current harmonics by the following two methods. The first one is to derive theoretically the return current harmonics with the motor current harmonics approximation. The second one is to simulate numerically a traction circuit including a inverter and a motor, in order to verify theoretically derived return current. Comparing the both results, we concluded the theoretical return current is enough accurate to be utilized for the future improvement of inverter harmonics current.