For ROtor Synthetic Aperture Radar (ROSAR) using the stepped-frequency waveform, an imaging method based on Range Migration Correction (RMC) for ROSAR is proposed. First, the signal model of ROSAR using stepped-frequency waveform is derived; Second, the range migration correction in frequency domain is proposed; Third, the high-resolution images are achieved by using the ω-k algorithm; Finally, the azimuth resolution performance is analyzed. Simulation results demonstrate the effectiveness and high resolution performance of the proposed method.