This paper focus on the mechanical properties of the surrounding rock (sandstone) in the chamber of Chongqing Panlong Pumped-Storage Power Station. Conventional triaxial compression instantaneous tests under different confining pressures were first performed using a servo-controlled triaxial testing machine for rocks, based on which a macroscopic elastoplastic constitutive model was developed and validated. The testing results show that with the increase of confining pressure, the compaction stage caused by the progressive closure of initial voids and microcracks becomes less and less obvious, and the nonlinearity disappears gradually.In addition, the peak strengths of rock specimen increase with the confining pressure, ranging from 84.8 MPa under uniaxial compression to 168.5 MPa under the confining pressure of 10 MPa. There presents a satisfactory linear relationship between peak strengths and confining pressures, from which we derive the material cohesion 15.2 MPa and the friction angle 52 degrees. Numerical results show that the proposed model containing a unified hardening/softening function can describe quite well the fundamental mechanical behaviors of the investigated sandstone, which demonstrates the feasibility of the proposed constitutive model.