The invention discloses a low excitation limit optimization setting method and system for an excitation regulator. Through the leading phase test of a generator unit, a leading phase capability valueof the unit under different active conditions is obtained; according to a manufacturer and a model of an excitation regulator, a calculation formula of the low excitation limit action characteristic is obtained, a low excitation limit action value is calculated, the unit reactive power is reduced till the low excitation limit action, if calculated deviation between the low excitation limit actionvalue and an actual low excitation limit action value exceeds a set range, the calculation formula is adjusted to recalculate the low excitation limit action value till the deviation is within the setrange so that the calculated low excitation limit action value is equal to or close to the unit leading phase capability value to set a low excitation limit fixed value; after setting the low excitation limit fixed value, the set low excitation limit fixed value is fine-tuned through the unit leading phase test till an error between the actual low excitation limit action value and the leading phase capability value is within the allowable range. The method is advantaged in that safe leading phase operation of the unit within the leading phase capability can be guaranteed.