In-situ true triaxial tests are conducted on thin-bedded marbled dolomite rock of the underground powerhouse tailwater tunnel of Wudongde hydropower station to investigate the deformation properties and strength properties of the engineering rock mass in unloading conditions. The sample is a square cylinder with 50 cm length，50 cm width and 100 cm height and contains 20–30 layers. Preloading is applied to avoid sample getting loosed in chipping process. Deformation tests are conducted under geostress level firstly and then simulating geostress variation pattern in the excavation process，the axial pressure is increased and then the lateral pressure perpendicular to bedding is step unloaded till sample failure. Some conclusions are drawn as follows：(1) Deformation is orthotropic. The ratio of the deformation modulus perpendicular to bedding to that parallel to bedding is 0.34–0.69. Beddings are opened and the lateral expansion coefficient is 0.52 while loading parallel to bedding and conversely，beddings are compacted and the lateral expansion coefficient is 0.25 while loading perpendicular to bedding. (2) There is large plastic deformation. The ratio of unloading residual deformation to loading deformation is 0.4–0.8 in the process of cyclic loading. (3) Loading deformation modulus is positively correlation with lateral compression and unloading deformation modulus perpendicular to bedding is negatively correlation with lateral compression. (4) Unloading deformation is nonlinear and the correlation between deformation modulus and unloading stress difference can be described by negative exponential function. (5) Sample failure is shear failure of bedding surface and sample volume is continuously and acceleratly expanded. (6) Mohr-Coulomb parameters of bedding surface are calculated based on shear failure mode of weak surface. Mohr-Coulomb and Drucker-Prager parameters of rockmass are calculated with presupposition that samples were homogeneous rockmass. The applicability of the parameters is analyzed.