It is different between the action value and the setting value of UNITROL5000 excitation system low excitation limit during generator's leading phase operation,and which leads to that the unit can't play its leading phase ability,and the line voltage operates much higher,it is not conducive to the safe operation of the power grid.This paper analyzes the low excitation limit action characteristics and influence factors of two different types of excitation system.Then an optimized setting method of low excitation limit is proposed.Finally,the feasibility of the tuning method is verified by an example.The low excitation limit setting method considers the generator's leading phase ability and the excitation-loss protection,the action value is as close as possible to the generator's leading phase ability and matches the excitation-loss protection reasonably after setting the optimized value of low excitation limit.The low excitation limit setting method provides a reference in the engineering practice for UNITROL5000 excitation system.