As underground engineering continues to advance deeper, the difficulty of breaking hard rocks is becoming increasingly prominent. Laser rock breaking is a new and efficient method of assisting in the crushing of hard rocks with great potential for engineering applications. To investigate the influence of size effect on laser rock breaking effect, a laser fracturing brittle hard rock simulation model was established based on ABAQUS finite element simulation software, and laser fracturing rock simulation experiments were carried out under different size conditions. Research has found that cracks first occur inside the sample under laser action, and then gradually propagate to the surface of the sample; The size effect does not have a significant impact on the temperature field of rocks, but has a significant influence on their temperature stress levels; For the rock breaking effect, increasing the sample size will delay the occurrence of rock cracks and reduce the crack propagation rate. The implementation phenomenon indicates that size effect is an important and non negligible factor affecting the effectiveness of laser rock breaking.