The invention discloses a thermal power generating unit primary frequency modulation optimal control method and system based on amplitude keeping. The method comprises the steps that when the absolute value of a unit rotating speed deviation signal delta n is within a preset range, a system scheduling assessment operating value is not reached, and system output is the load compensation quantity delta P1 generated after the unit rotating speed deviation signal delta n is subjected to compensation according to a primary frequency modulation compensation quantity function f(x); when the absolute value of the unit rotating speed deviation signal delta n is beyond the preset range, the system scheduling assessment operating value is reached, final output quantity of the system is determined according to the positive/negative value of the unit rotating speed deviation signal delta n, and the duration of the final output quantity of the system is determined according to preset retardation time. The thermal power generating unit primary frequency modulation optimal control method and system based on amplitude keeping have the advantages that the practical problems of a poor assessment index of primary frequency modulation of a thermal power generating unit and the like can be solved, the requirement of a power grid for primary frequency modulation of the unit can be effectively met, and the demands for frequency modulation and peak regulation are met.