Hydraulic fracturing is the main exploitation means of low permeability and ultra low permeability oil and gas reservoirs，and its effect on stimulated reservoir is affected by many factors. The stimulated reservoir area is an important index of evaluating the fracturing effect，but its main influencing factors are not clear. Therefore，a mechanical model of multi-cluster staged fracturing of horizontal well was established. The numerical simulation analysis technique and scalar damage variable model were used to simulate the stimulated reservoir area and the initiation and propagation of hydraulic fractures for multi-cluster staged fracturing. The results show that the interference between fractures is one of the important factors affecting the reservoir stimulation effect；for the multi-cluster staged fracturing，the increase of stage spacing can improve the stimulation effect；in the case of low stress difference，increasing injection rate makes the stimulated reservoir area expand by 32.1%. When the fluid viscosity is high，improving injection rate makes the stimulated reservoir area increase by 70.6%. High viscosity fluid，large injection rate and low stress difference can effectively improve the stimulated reservoir area.