Abstract: The response time is one of the critical technical parameters of resistance temperature detectors. For the current immersion method, the off-line measurement is usually carried out in the laboratory, which cannot reflect the dynamic response characteristics of RTD under actual installation conditions. An in-situ response time measurement device based on loop current step response（LCSR） has been developed. Consequently, the principle, method and results of response time measurement are compared with those of immersion method. Under the test conditions of flow velocity（0.2-0.6） m/s, the measurement results of the two methods at different medium flow velocities are compared, and the influence of flow velocity on the response time of the RTD is analyzed too. The results show that the response time decreases with increasing flow rate. A deviation within ±10%, as well as similar variation trend and dispersion, can be observed between above mentioned methods, meeting the requirements of the performance monitoring test standard for instrumentation channels important to safety（NB/T20069-2012）. By adopting the internal current step mode, LCSR effectively overcomes the difficulty of immersion method in simulating the high-speed flow of medium. It presents measurement consistency with immersion method during comparisons under laboratory condition, and can be further extended to various applications for in-situ measurement of the response time of RTD in nuclear power plants under different installation conditions.