Abstract: This paper studied the determination of the optimum model from various visco-elasticconstitutive models. In this paper, rock mass, as a complex system, deforms gradully with timeunder construction. Based on the principle of complex system model, this kind of deformation isregarded as an output of the system. The paper derived a method to determine the optimummodel under specific conditions from a general formula of visco-elastic constitutive model byusing least square non-linear optimum method and fitness examination. The effectiveness andfeasibility of the method are confirmed by an example, which shows, through verifying andcorrecting by using new measured data, this method may be used in prediction of engineeringbehaviors, and a combination of theory and practice is obtained.