System distortion is inevitable in wideband imaging radar, which needs to be compensated precisely.In traditional wideband imaging radar for space surveillance, a linear frequency modulation signal is transmitted and the echoed signal is received with dechirping processing. In this case, the system distortion changes with the target range, making it hard to compensate the distortion. However, the direct intermediate frequency sampling(DIFS) signal maintains the complete system distortion and avoids the range variant distortion in the dechirped signal. Therefore, it is more convenient to perform system compensation in DIFS signal. In this paper, the distortion factors affecting the wideband radar systems are introduced. Then, the influence of the amplitude phase distortion on the focusing quality is analyzed in detail. Finally, a system compensation method in the frequency domain based on least squares estimation is proposed. In the proposed method, the compensation vectors are extracted from the calibration tower echoes for DIFS compensation. Inverse synthetic aperture radar imagery of targets can be achieved with improved focus quality. Simulations and real-data experiments confirm the effectiveness of the proposal. System distortion is inevitable in wideband imaging radar, which needs to be compensated precisely. In this case, the system wideband imaging radar for space surveillance, a linear frequency modulation signal is transmitted and the echoed signal is received with dechirping processing.以, the direct intermediate frequency sampling (DIFS) signal maintains the complete system distortion and avoids the range variant distortion in the dechirped signal. Therefore, it is more convenient to perform system compensation In this paper, the distortion factor affecting the wideband radar systems are introduced. Then, the influence of the amplitude phase distortion on the focusing quality is analyzed in detail. Finally, a system compensation method in the frequency domain based on least squares estimation is proposed. In the proposed method, the compensation vectors are extracte d from the calibration tower echoes for DIFS compensation. Inverse synthetic aperture radar imagery of targets can be achieved with improved focus quality. Simulations and real-data experiments confirm the effectiveness of the proposal.