Although range Doppler (RD) algorithm is widely used in synthetic aperture radar (SAR) imaging, the poor images produced by this algorithm fail to satisfy the needs of practical applications. In order to solve the key technical problems that restrict the imaging quality of traditional methods, a high resolution SAR imaging algorithm based on optimal sampling length is proposed. The proposed algorithm designs the calculation formula of the initial sampling length according to the imaging parameters of the range direction and azimuth direction of SAR. Combining the obtained initial sampling length and the evaluation criteria of SAR image quality performance with the curve extremum, the optimal sampling lengths of range direction and azimuth direction within a reasonable scope of changes are determined. The experimental results of point target imaging of airborne SAR and the measured data of space-borne SAR show the effectiveness of the proposed algorithm.