Signal distortion may happen in conventional accoupled interferometric radar when measuring slow target movements. Existing solutions to preserve signal integrity add to hardware complexity and are at extra cost. In this paper, a digital post-distortion (DPD) technique is proposed to compensate the signal distortion in baseband digital domain. For the first time, at no extra hardware cost, an ac-coupled interferometric radar can be used to measure slow movements without losing any signal information-even those with stationary moment. The DPD technique is realized by applying a compensation function, which is the inverse of the baseband system response and obtained in hardware calibration, to the digitized baseband I/Q signals. Both simulation and experimental results show that the proposed technique is robust to recover distorted signals.