The compensation relationships among the adjustable variables and those between surface errors and the adjustable variables in off-axis three-mirror system are analyzed. The decenter variable in the adjustable variables often brings the same kind of aberration as the position offset of the tilt variable does, so they are of considerable relativity. Based on the theory of offset aberration, the relationship between decenter variables and tilt variables is obtained by compensating the primary aberration caused by both the decenter variables and the tilt variables. The adjustable variables and surface errors are correlated by modeling the relationship between surface errors which is represented by Zernike polynomials and the primary aberration. The precondition of computer simulation for compensation is to adjust the decenter, tilt and transverse displacement of the optical elements when given a certain value of surface error. The result shows that the wave front performance degradation of compensated system is