Abstract: To study the rock mass damage variable caused by joints under osmotic environment, the damage variable calculation formula of the rock mass caused by the osmotic environment is firstly deduced based on the connection of the increment of additional strain energy in fracture mechanics and the emission of damaged strain energy in damage mechanics. Secondly, according to the theory of fracture mechanics, the calculation methods of the stress intensity factor Ⅰand Ⅱ under osmotic pressure are obtained for non-penetrating joints. Then, the calculation methods for the damage variables of non-persistent joints under osmotic pressure are established. Finally, the calculation formula is verified and analyzed. Under the condition of osmotic pressure, the damage variable gradually increases with the half-length of joints. With the increase of osmotic pressure, the damage variable of rock mass increases gradually. The damage variable decreases with the increase of internal friction angle and joint surface softening coefficient.