By introducing fractional calculus theory to the rock creep model, pronounced effect was obtained in describing the creep property of rock mass. However, the orders of the conventional fractional viscous body almost are almost constant, so a single element cannot be used to simulate the attenuation stage, steady stage or the accelerating stage of rock creep. According to the fractional calculus theory, a modified fractional viscous body with unsteady differential orders was developed. Then, through connecting it and the ideal viscoelastic body that introduced the fractional calculus with the traditional elastic body, a new nonlinear viscoelasto-plastic creep model for rocks was established. In this paper, the creep equation of the model was deduced, and the rationality of which was verified by using creep test data of rocks. What's more, the paper made a comparison between the proposed model and the Nishihara model, the HoHai model, respectively. The results indicate that the fitting curves and test curves conform very well to each other. The proposed model has high accuracy and rationality and can well reflect the whole stages of rock creep process.