Abstract: Karst rock mass are widely developed in Southwest China. Due to the karstification of rock mass,the physical and mechanical properties of the karst rock mass are different from fractured rock mass in nature. The research has great significance on mechanical properties of karst fractured rock mass. It provides reasonable basis for engineering design,accurate and reliable parameters for the evaluation and prevention of karst collapse and landslide disasters,and modifies the stability evaluation model. Karst fractured rock mass in nature can be considered as rock mass with initial damage. In order to study the damage characteristics of karst fissure rock mass,this paper takes karst rock mass in Guizhou Province as research object. The Weibull distribution and equivalent strain principle is used to establish the damage evolution equation of karst fractured rock mass with initial damage. Meanwhile,the theory of rock strain strength is used establish the constitutive equation of karst fractured rock mass damage under uniaxial compression. The two equations provide a basis for studying the characteristics of dissolved rock mass with initial damage. Furthermore,the discrete element numerical software is used to carry out uniaxial compression numerical tests to study the evolution characteristics of karst fissure rock specimens under uniaxial compression conditions. In order to determine the microscopic parameters of the limestone,the uniaxial compression test of intact rock and the direct shear test of the structural plane are carried out. In addition,the random distribution of fracture in rock mass is obtained using field survey and statistics method. Finally,fractured rock mass model is constructed using particle flow discrete element software according to experiments and field investigation. The results show that the initial damage of karst fractured rock mass mainly includes erosion damage and crack damage. The initial damage of karst fractured rocks increases with the increase of the dissolution rate,and the final increase rate tends to flatten. The damage evolution curve of karst fractured rock mass all show the "S" behavior of slowly rising firstly,then rapidly growing,and finally slowly increasing to a damage value of 1. For fractured limestones with different karst rates,the compressive strength generally decreases as the dissolution rate increases. As the karst rate of fractured rock masses increases,the destruction of fractured limestone is mainly concentrated around the karst cavities,and the fragmentation characteristics after failure are also weakened. Local damage of sample can cause entire damage. In the initial stage of loading,the karst fractured limestone is mainly destroyed along the fractures. When the fractures are partially destroyed,the fractured limestones have a certain strength. While the residual rock blocks are destroyed,and the sample can be destroyed wholly. The heterogeneous characteristics of karst fissure rock masses leads to the failure originating from the dissolved pores and fractures with initial damage. Then the fractures undergo macroscopic damages after four stages of fissure initiation,expansion and shear,increase in number and length,and fissure penetration. The research results provide a scientific basis for parameter selection,optimal design and construction of engineering rock masses in karst areas.