The invention is applicable to the technical field of nuclear power, and provides a method and device for preventing cold and excess pressure in a nuclear power plant. The method for preventing cold and excess pressure includes the steps of: monitoring the working state of a high-pressure safety injection system; if the high-pressure safety injection system is in operation, acquiring the current temperature of the inner wall of a pressure vessel and a setting value curve of a voltage regulator safety valve, and determining a current setting value of the voltage regulator safety valve accordingto the current temperature and the setting value curve of the voltage regulator safety valve; regulating the setting value of the voltage regulator safety valve according to the current setting value; detecting the current pressure of the pressure vessel under the current temperature; and if the current pressure is greater than the setting value, switching on the voltage regulator safety valve. According to the method and device for preventing cold and excess pressure in the nuclear power plant, continuous rising of the pressure of a primary circuit can be avoided, so that the pressure whichis applied to the pressure vessel is controlled within an allowable range, and the pressure vessel can be prevented from brittle fracture.