||This paper presents a new constitutive model for the description of elastoplastic behaviour of a porous rock. In the first section, a brief summary of the experimental investigations is presented. The data obtained show that an important coupling exists between elastic and plastic deformation, and that the confining pressure has a great influence on the yielding process of the rock. In the second section, firstly, a plastic internal variable is defined, which is able to record the influence of the stress state on the progressive yielding process. Then, based on the plastic internal variable, the respective evolutions of the subsequent yield surface, dilatancy angle and elastic parameters are studied, and the applicability of the generalized orthogonal flow rule which connects the irreversible strain increment and the yield function are analyzed. The results show that the generalized orthogonal flow rule is not suitable for the porous rock during plastic deformation. Then, based on the experimental evidence, a general formulation of the model is presented. In the third section, simulations of the conventional triaxial compression tests are performed, and the results show that the proposed model is indeed capable of capturing the main features of the mechanical behaviour of the rock. Finally, the model is extended to simulate unloading confining pressure tests, and for which it gives reasonable results.