The stepped-frequency radar can measure the velocity between radar and target as while as obtain high range resolution. The Doppler shift of moving target may make its high resolution range profile move away from high resolution imaging region of stationary target. Consequently stepped-frequency radar possesses the ability of detecting dim target in clutter, which is suitable for attacking low-altitude dim target with low velocity. In this paper, the distribution of clutter is analyzed in frequency domain firstly. Then the design method of signal parameters is put forward based on the analysis in frequency domain. Finally, the simulation results demonstrate the effectiveness of the method.