The traditional creep model is difficult to characterize the complete creep behavior of rock. In order to reveal the unloading creep mechanical properties of granite, the high-potassium calc-alkaline I-type granite of a pumped storage power station was taken as the research object, and the triaxial unloading creep test under the constant axial pressure unloading confining pressure stress path was carried out. The creep strain characteristics and rock damage evolution law of granite during unloading process were analyzed. Based on the fractional calculus theory, a variable coefficient non-definite fractional unloading creep damage model which can accurately describe the whole process of granite creep was constructed and extended to three-dimensional stress state. The results show that in the triaxial unloading creep test, with the decrease of confining pressure, the total axial creep of each stage of the sample increases continuously, and the total axial strain is dominated by instantaneous elastic strain. The rock exhibits strain softening characteristics and is brittle failure when destroyed. The parameter identification and fitting of the triaxial unloading creep test data were carried out by using the variable coefficient unsteady fractional unloading creep damage model. The fitting results show that the model can better describe the creep damage evolution characteristics of the high potassium calc-alkaline I-type granite in Yanshan period. The research results can provide experimental basis and theoretical guidance for the long-term stability research and support scheme design of underground caverns in similar areas.