Abstract: Visualized experimental techniques are increasingly used in the measurement of nuclear reactor system parameters. Based on the characteristics of laser induced fluorescence (LIF) technique, the difficulties and solutions of the LIF technique were introduced in this paper. And the temperature distributions downstream of spacer grid in rod bundle channel under the steady flow and pulsating flow were analyzed. The results show that the application range and measurement accuracy of LIF technique can be improved by studying the optical properties and dye characteristics. At the same time, post-processing technology can be used to obtain more accurate temperature field distribution. The full-field temperature distributions downstream of spacer grid in rod bundle channel under steady flow and fluctuating flow conditions were obtained. Spacer grid can significantly enhance flow mixing and improve heat exchange capacity. Temperature distribution is also affected by fluctuations in velocity. In summary, the LIF technique can achieve the full-field measurement of the temperature distribution in the rod bundle channel. According to the temperature distribution characteristics, the performance of the spacer grid can be evaluated.