The invention belongs to the nuclear electrical field and provides a power oscillation suppression method and device for the main power transformer power transmission power grid of a nuclear power plant. The method includes the following steps that: frequency calculation is carried out according to three-phase voltage and three-phase current in the main power transformer power transmission power grid of the nuclear power unit so as to obtain frequency change; high-frequency harmonic waves in main power transformer power transmission are filtered out; and amplitude limiting is performed, so that output will not exceed an amplitude limiting constant value when the frequency change is larger than a frequency amplitude limiting design value, and therefore, a PSS (power system stabilizer) will not output power oscillation signals. According to the method and device of the invention, after the high-frequency harmonic waves in the main power transformer power transmission are filtered out, amplitude limiting is performed, and therefore, the output will not reach the amplitude limiting constant value, and therefore, the output can be still in normal amplitude when the frequency change is larger than the frequency amplitude limiting design value, and the PSS will not output oscillation fluctuation signals, and the problem of the abnormal working condition output of the main power transformer power transmission in the frequency measurement link of the PSS can be solved, and negative effects on power fluctuation caused by the output of an excitation regulator can be avoided, and the hidden risks of the power oscillation of the unit can be eliminated.