The present invention relates to a shear test device and test method of rock mass discontinuities under constant normal stiffness condition. The device includes: a base used for placing a test piece; a loading framework fixedly connected with the base; a shear loading system used for applying a shear force to the test piece; a normal loading system used for applying a normal pressure to the test piece; a normal displacement monitoring system used for measuring the magnitude of normal displacement of the test piece in real time; and a computer control system used for receiving the data of the normal displacement of the test piece in real time, and constantly adjusting the normal pressure applied by the normal loading system to the test piece according to the principle that the normal stiffness of the test piece is unchanged.