［Purposes］ The complex stress-strain relationship of coal rock under high stress is studied by establishing a stepped loading and unloading constitutive model. ［Methods］ The damage constitutive model of coal rock is established combined with the double-shear unified strength theory, in which the hypothesis that the micro-element strength of coal rock obeys the Weibull probability density distribution is adopted. The constitutive model is validated by using coal rock experiments. Then, the obtained experimental stress-strain curve is fitted to obtain the evolution law of parameters, and the sensitivity analysis of these parameters is carried out.［Findings］ During the loading and unloading test of coal rock, the inside plastic deformation of coal rock is gradually accumulated and the area of the hysteresis loop keeps increasing. An increase in loading and unloading level leads to the hardening of coal rock, an increase in the inside damage and brittleness of coal rock, and a decrease in micro-element strength of coal rock.［Conclusions］ The damage constitutive model of coal rock based on the double-shear unified strength theory established in this study can accurately express the stress-strain relationship of coal rock under the conditions of stepped loading and unloading, which is of some theoretical and practical significance.