The bending theory of bending mechanism loaded at four point of torodial focusing mirror in synchrotron radiation is introduced,and a model of bending mechanism is established. A nonlinear error function be-tween ideal arc and factual achieved,the optimized parameter l to minimize the error are given,based on the solution of error function using the technology of CAE. The practical application in torodial focusing mirror of BESRF－3B3 beamline shows that this system is successful.