In order to further analyze the mechanism of microwave rock breaking, the damage evolution law and constitutive behavior of hard rock under microwave irradiation are studied, firstly, the elastic micro element hypothesis of rock is carried out. Combined with the micro element strength criterion and three parameter Weibull distribution, the damage evolution equation, constitutive model and parameter determination formula of hard rock after microwave irradiation are derived, Then, the model is verified by ultrasonic testing and uniaxial compression test results of granite irradiated with different microwave power. The results show that the theoretical curve of the model is in good agreement with the experimental curve, which shows that the model can reflect the stress-strain process of granite fracture, the physical significance of the parameters of the model and the influence law on the model are clear, and there is a certain deviation in the fitting after the peak, but the model can still basically reflect the weakening law of rock after microwave irradiation. The research results can provide some reference value for the related calculation and numerical simulation of microwave irradiated rock.