The invention discloses a method for preventing thermal power of a reactor core from exceeding a limiting value in a nuclear power plant. The method comprises the following steps: acquiring a measured value of high pressure cylinder inlet pressure in a steam turbine, and performing proportional-integral calculation on the measured value of high pressure cylinder inlet pressure and a constant value of the high pressure cylinder inlet pressure in the steam turbine; and acting the proportional-integral calculation result and a constant steam flow value on a power control loop, and controlling the thermal power of the reactor core not to exceed the limiting value by controlling the opening of a high-pressure cylinder valve by virtue of a steam turbine adjustment system according to the proportional-integral calculation result and the constant steam flow value. For a long time, the problem that the thermal power of the reactor core exceeds the limiting value cannot be effectively solved, so that potential hazards of the reactor core easily occur under instantaneous conditions. The constant value of the high pressure cylinder inlet pressure and the constant steam flow value are reasonably set, so that the thermal power of the reactor core is effectively and automatically controlled without exceeding the limiting value by controlling the opening of the high-pressure cylinder valve by virtue of the steam turbine adjustment system according to any one constant value of the constant value of the high pressure cylinder inlet pressure and the constant steam flow value, and hazards of the reactor core and the nuclear safety problem caused by that the thermal power of the reactor core exceeds the limiting value are solved.