Based on the analysis of existing dechirp approaches, this article provides a modified high efficient digital dechirp approach on synthetic aperture radar(SAR). Under the premise that the pulse width is longer than the main irradiation area, sampling rate lower than the bandwidth of transmitted signal can be applied to receive the echo signal in range with dechirp. The dechirped signal is then compressed after FFT without aliasing. The method highly reduces the sampling rate, thereby decreases the requirement on ADC devices and then reduces the pressure upon following signal processing and storage. It provides an effective way to realize the miniaturization of SAR. The article puts forward theoretical analysis and mathematical derivations of this improved digital dechirp approach, and the accuracy and effectiveness of the method have been verified by the formulation and simulation.