Abstract: Critical heat flux(CHF) plays an important role in reactor safety, and observing temperature rising rate can effectively judge the occurrence of CHF. Haar Wavelet function is used to analyze the temperature rise curve data of CHF experiment in natural circulation narrow channel. Through the decomposition and reconstruction of the temperature signal, it is found that the conclusion of wavelet analysis is in good agreement with the analysis results of temperature rise data obtained by the experiment, and the location and time of CHF can be accurately judged.