Abstract: First of all, weakened material is generated by introducing strength reduction factor(SRF) into bonded-particle model(BPM), hereafter, uniaxial compressive strength tests on weakened specimens are performed. The results indicate that: the weakening effect both reduces uniaxial compressive strength of the specimen and leads to a gradual decline in elastic modulus of the specimen, which is consistent with findings of related laboratory tests. In order to further examine the effects of the proposed weakening method, a numerical model for direct shear tests on weakened rock joint is developed by means of a computer program, which is written by a built-in language named FISH in particle flow code (PFC); and then direct shear tests are conducted at various normal stresses. Tests results indicate that: the shear character of weakened joints corresponds well to that of real rock joints, in addition to the dependencies of shear strength and peak dilatation angle on normal stresses are also conform well to the classical JRC-JCS model. It is suggested that the rock strength reduction method adopted in this study could preferably simulate the effect of strength reduction of rock material. Finally, investigation of evolution feature of microcracks within specimens shows that: attenuation of joint wall will lead to a rapid growth and development as well as rapid expansion of distribution range of microcracks, and will also result in a rapid increase in the proportion of shear cracks in specimens. As a result, the reason for a weakened joint is more susceptible to arise shear failure is revealed from mesoscopic angle. Research results in this article may provide a reference for the study of shear strength of rock joints，and for the study of stability of large-scale rock slopes.

Key words: weakened joint, clumped-particle model(CPM), shear strength, JRC-JCS model, microcrack