SISAR imaging algorithm is a kind of technology to extract the profile of a target from the forward scattering signal. The imaging accuracy depends on the target coordinate estimation precision, but few published papers about the SISAR imaging accuracy are available. In this paper, the relationship between the imaging error and coordinate estimation errors is analytically derived with the forward scattering signal model; under the assumption of small diffraction angles, the quantitative requirements for coordinate estimation precision are presented; finally, impacts of coordinate estimation errors on SISAR imaging accuracy are verified via simulation results.