The invention discloses a method for preventing the ultra-low frequency oscillation of a hydropower generating set during active power regulation. According to the method, oscillation monitoring is performed according to the active power fixed-value adjustment and primary frequency modulation of the hydropower generating set; a fixed-value adjustment monitoring module applies a static characteristic that an active power set value usually keeps unchanged during fixed-value adjustment, and a primary frequency modulation monitoring module adopts a grid frequency as a reference, and therefore, theactive-power ultra-low frequency oscillation of the hydropower generating set which is adverse to the stability of a power system can be effectively distinguished. A judgment logic and B judgment logic are designed for primary frequency modulation with complicated working conditions; only when the two kinds of logic both consider that the primary frequency modulation is not conducive to the stability of the power system, the primary frequency modulation function is automatically withdrawn, otherwise only an alarm is issued, and operation personnel decide whether the primary frequency modulation function is to be withdrawn; and therefore, the possibility of the misjudgment, and mistakenly withdrawing the frequency modulation of a real-time control system can be eliminated, and the reliability of the function is improved.