To improve the precision of a bending mirror and to reduce the influence of a horizontal deflected bender and the mirror weight on the bending mirror,the factors that caused the slope errors were studied and a gravity compensation scheme for the horizontal deflected bender was presented. Firstly, the slope errors were studied on the basis of bending theory. Aiming at the bending focusing mirror used in the beamline, the main factors that affected its precision were analyzed by finite element method. Then, according to the characteristics of the horizontal deflected bender and the results of slope error analysis，a scheme was carried out to compensate the slope errors. The results indicate that the gravity of the mirror and a cooling device mainly had influence on the cylinder mirror generatrix, while the gravity of the driven-rod mainly affected the tangential slope error. After compensation，the slope error rms of cylinder mirror generatrix is reduced from 13.14 μrad to 015 μrad and the tangential slope error rms is reduced from 8.21 μrad to 0.86 μrad. It verifies that the slope error analysis and gravity compensation scheme proposed is effective, which improves the precision of the bending mirror greatly.