It is impossible to remove the high order aberrations of minimum aberration project objective only with adjustable structures,which is a bottleneck to restrain further optimization.Ion beam figuring (IBF)instrument is widely used in optical lens manufacturing.IBF can well control low and middle frequency figuring errors to obtain high accuracy figuring polish.The wavefrint error of projection lens small scale model is analyzed using fringe Zernike polynomials,which contains a large amount trefoil aberration.One surfaces of the objective is chosen to remove designated figures using IBF to compensate trefoil aberration of the whole system.Simulation and experiment results show that trefoil aberration is well controlled,and the wavefront error is reduced from 29.6 nm (RMS) to 12.7 nm (RMS).The quality of objective is greatly improved,which proves the corrextness and applicability of this trefoil aberration compensating method.