Abstract: There is a certain gap between the main equipment and its support in a nuclear power plant nuclear island to allow the free thermal displacement and thermal expansion caused by the change of temperature and pressure of the reactor coolant system. The stability of gap is important for the safe operation of the nuclear power unit. Therefore, the support gap needs to be measured and evaluated in each refueling cycle of the in-service nuclear power unit, which takes a long time and has high radiation risk. In this paper, the influencing factors of support gap composition are analyzed. Combined with the historical data of gap measurement and engineering experience, the concept of gap stability and its acceptance criteria are proposed, and the evaluation process of gap stability is determined. Based on the measured data of the support gap of the voltage stabilizer in a running nuclear power plant, it provides a basis for shortening the period of gap measurement. The method can shorten the cycle of shutdown and refueling, reduce the radiation dose of the overhaul and measurement personnel, and ensure the safe operation of the unit and improve the economy.