Micromechanics is used in the study of mechanics character of slate. A constitutive model of slate under compression and seepage is established. The process of swelling distortion caused by shearing and seepage are considered for the equations during establishing the constitutive models. The frictional sliding and mode-Ⅱ self-similar growth of closed microcracks under compression and their influences on the mechanical properties of rock are studied in detail. The model is validated by experiment.