Abstract：The rock strength gradually degraded with time and stress if applied load exceeds a certain yield stress level in creep process；and this damage law could be quantified with the reduction in the basic mechanical parameters of rock，i.e. elastic modulus E，cohesion c and internal friction angle j. Based on the creep experiments of mudstone obtained from Qinyuan coal mine in Baoji under conditions of eight different stress levels and three different times，the variations of mechanical parameters E，c and j were measured in the process of mudstone creep by using single specimen method. Then，the coupling functions，that the mechanical parameters E，c and j decayed exponentially with increases in stress level，long-time strength and time，were established through test data. Moreover，the mudstone common expression of damage law of mechanical parameters was obtained by analyzing the characteristics of coefficients in coupling functions，in which the parameters had clear physical meanings and simple testing methods. This study could provide the foundation for further establishment of non-stationary rheological constitutive model.