Abstract: Stepped-frequency is a kind of high range resolution waveform, which is used to obtain broad effective bandwidth by sequentially changing of carrier frequency over sub-pulses in one burst, so it reduces requirements on instantaneous bandwidth and receiver. However, this waveform is so sensitive to radial velocity between radar and targets that the loss of range resolution and loss of SNR are brought to SAR system. In the paper, the effect of radial velocity on synthetic range profile is discussed thoroughly first, then two important parameters, target range shift and range spread, are defined in terms of the phase of synthetic range function. Furthermore, the enhancement of the range sample resolution only makes the synthetic range profile present more details, but it will not change the range resolution.