Abstract: Fractured rock mass with densely distributed joints is often encountered in geotechnical engineering field. So it is necessary to develop a reasonable constitutive model to conduct studies on mechanical behavior of such kind of rock mass. A mixed multi-weakness planes softening constitutive model is established for the rock mass with less than three sets of joint weakness planes, which is considered as a macroscopic composite material in a broader sense. The rock mass satisfies composite the Mohr-Coulomb criterion with tension cutoff or the Hoek-Brown failure criterion by setting yield factor. Joint plane with bilinear characteristics follows the composite Mohr-Coulomb criterion with tension cutoff. Softening parameters, which are variables correlated to plastic strain and tensile strain, are used to describe the softening characteristics of materials. Based on the secondary development platform of constitutive model in FLAC3D, this mixed softening model has been successfully developed and embedded. It is verified by simulating the uniaxial compression test on rock mass that is obliquely cut by double joint planes. The calculated results agree with the related tests fairly well, and satisfy the macro-destruction mechanism of rock. Thus, it is shown that this model is correct and can be applied in practical projects.