The invention discloses a generator set primary frequency modulation dynamic compensation method and system based on steam enthalpy drop. Real-time parameters of a temperature, a pressure and flow ofadmission and various steam extraction and steam exhausts of a steam turbine are acquired; a steam enthalpy value parameter table is queried to obtain an enthalpy value of each parameter of the steam;a difference between work done by steam extraction of the steam turbine and work completely done for enabling the steam to flow out of a steam exhaust port is calculated; and by conversion, the workdifference is compensated to high-pressure admission of the steam turbine so as to complete correction on set primary frequency modulation load compensation. According to the invention, advanced and reasonable compensation of a set primary frequency modulation instruction is implemented, and the problem that primary frequency modulation load compensation of an extraction-condensation type unit generator set cannot reach the design requirements when a power grid frequency is fluctuated in a range of a small amplitude is solved, so that a technical index of primary frequency modulation of the generator set is improved.