In this paper, based on the phase field fracture model, the size effect of 4 groups of concrete with initial cracks is studied by using the three-point bend zone test method, and the crack growth trend, stress distribution in the X direction and crack opening-displacement curve are discussed when concrete cracks occur, and compared with the experimental results. A three-point bending finite element model with different dimensions is established by selecting different depth values and considering different notch depths. The finite element method is used to simulate the three-point bending fracture of concrete and analyze the size effect of concrete fracture. The simulation results are in good agreement with the experimental results in the literature, and the results show that the phase field fracture model can effectively analyze the size effect of concrete fracture. The phase field model can effectively combine the crack initiation criterion based on strength, the propagation criterion based on energy and the fracture path criterion based on variational principle, so that it can be used to analyze the crack evolution of concrete with initial defects and without initial defects, which can provide a useful reference for the mechanical failure prediction and protection of concrete.