To locate airborne synthetic aperture radar (SAR) image with high accuracy, a fast inertial navigation error estimation method is proposed by using advantage of sub image back projection (BP) imaging. The relationship between inertial navigation error in position, velocity and positioning error under BP imaging model is investigated. The observation equation of positioning error is obtained by approximating the relationship in linear. Estimations of inertial navigation error in position and velocity can be obtained by least square method. Monte Carlo tests are processed under Gauss observation errors with the same mean and variance. It is illustrated that the proposed method possesses a lesser error of mean square compared with the conventional range-Doppler (R-D) model based estimation method.