Chip size of LED greatly affects the current density distribution inside the LED chip. By studying the relationship between chip size of LED and current density distribution, it can provide key reference for LED chip designing and optimizing. This paper theoretically discusses the relationship between LED chip size and current distribution, and simulating the effect of chip size on LED current spreading capability using COMSOL MULTIPHYSICS software. The simulation results show that the current expansion capability of the large-sized LED chip is relatively poor, the current crowding phenomenon is more serious than the small chip. The current density around the electrode is significantly higher than other regions in the large-sized chip. However, the small-sized current density distribution is relatively uniform. The simulation also shows that the current expansion capability tends to be stable when the LED chip reaches a certain size.