The fracture toughness is the most important indicator in the rock fracture mechanics, which represents the ability to resist the crack propagation or the resistance needed to overcome new formations of crack surfaces. In this paper, based on the particle flow theory, the Particle Flow Code (PFC) model of rock with a pre-existing joint is constructed by using the parallel-bond model and the joint model. The failure process and the crack propagation path during the Brazilian disc(CSTBD) test are numerically simulated by using the particle flow code in two dimensions (PFC^2D), and the fracture toughness of the rock model is measured based on the simulated test data and the energy balance. Comparisons between numerical and experimental results show a good agreement, which indicates the validity and practicality of this method.