In order to study the size effect of the shear characteristics of complex fractured rock mass, taking PDC3 adit of Songta Hydropower Station in Nujiang, Tibet as an example, on the basis of on-site fracture information collection, based on PFC^3D software, a 3D fracture network was generated by statistical method and Monte Carlo principle, and the equivalent rock mass was constructed. The shear strength, internal friction coefficient and cohesion of fractured rock mass were obtained by large-scale direct shear test, and the size effect of the fractured rock mass was studied and the REV size under different parameters was determined. Then, the related parameters and sample sizes were fitted by nonlinear regression to explore their functional relationship. The results show that the shear strength, cohesion and internal friction coefficient tend to be stable with the change of sample size, and the REV obtained from different parameters are different. Comprehensive analysis of REV size of different parameters shows that the REV size of fractured rock mass for the shear characteristics in the study area is 11 m×11 m×11 m. By fitting the function relationship, there are exponential function relationships between shear strength, cohesion and internal friction coefficient and sample size. These results provide a basis for studying the mechanical REV of complex fractured rock mass and reasonably determining the mechanical parameters.