As an important geological environment medium for engineering construction in southwest China, basalt generally contains hidden microcracks at a micro-scale, which leads to significant size effect on its mechanical properties, and relevant research is lacking and unsystematic. A synthetic rock mass model based on the combination of a micro-discrete fracture network method and a finite-discrete element method is used to systematically explore the size effect of the mechanical properties of basalt rock blocks with hidden microcracks. The results showed that: (1) the fracture angle is uniformly distributed, and the fracture length conforms to the logarithmic normal distribution, with an areal fracture intensity P20 of 0.00025/mm