With the aim of simulating the behavior of saturated sand with initial static shear (i.e., sloped ground) undergoing undrained cyclic loading, which leads to liquefaction and large cyclic shear strain development, an elasto-plastic constitutive model which can describe both monotonic and cyclic torsional shear behaviors of saturated sand under drained and/or undrained conditions is presented. It can simulate qualitatively the stress-strain relationship and the effective stress path, even after the specimen enters fully liquefied state. To verify its effectiveness, the proposed model is employed to simulate the results of a series of hollow-cylindrical torsional shear tests on loose Toyoura sand specimens with initial static shear stress under stress-reversal and non-reversal loading conditions.