The invention discloses a control method and system for primary frequency modulation of a thermal power generation unit. The method comprises steps as follows: frequency modulation capability parameters of a steam turbine control valve are acquired; according to the frequency modulation capability parameters of the steam turbine control valve, a control valve frequency modulation threshold value of the power generation unit in a frequency modulation range of plus-minus 11 r/min is acquired; whether grid frequency exceeds the control valve frequency modulation threshold value under current working conditions is judged; if the grid frequency exceeds the control valve frequency modulation threshold value, primary frequency modulation is performed in a mixed frequency modulation manner for controlling the opening degree of the steam turbine control valve and controlling the opening degree of a feed-water bypass; and if the grid frequency does not exceed the control valve frequency modulation threshold value, primary frequency modulation is performed in a control valve frequency modulation manner for controlling the opening degree of the steam turbine control valve. The opening degree of the feed-water bypass and the opening degree of the steam turbine control valve are controlled for simultaneous operation, the requirement for primary frequency modulation of the unit is met, the frequency modulation manners are selected according to actual conditions, the opening degree of the steam turbine control valve is involved in both manners, the requirement for the rapid response to primary frequency modulation can be met, and the problem that primary frequency modulation responding capability is reduced due to restrictions of boundary conditions in the condensation water throttling frequency modulation process is solved.