The response time is a key factor affecting the dynamic measurement error of resistance temperature detector. The loop current step response (LSCR) method can realize the in-situ measurement of the response time of in-service resistance temperature detector. But the variable Joule heat, internal and external influencing factors on thermodynamic heat transfer processes, the conversion relationship and order are still to be studied. Based on COMSOL Multiphysics software, a multi-physical field coupling model was established to numerically study the coupling process of electric current excitation, temperature field, and external flow field in the LCSR, focusing on the mechanism of variable Joule heat generation of resistance wire involved. Consequently, the conversion relation between LSCR method and plunge test was established. The LCSR dynamic response curve was transformed by the third-order relation equation, and compared with the plunge test, the relative error of response time was only 1.51%. A study on the factors affecting LCSR response time was conducted, and the effects of external fluid flow rate, internal filling material and air layer on the response time were analyzed in depth. LCSR and plunge test validation experiments were simultaneously conducted. The comparison shows that the relative error between the simulation results and the experimental results of the response time of LCSR method is 2.46%. The LCSR experimental data were converted by the relation equation, and the results are compared with the experimental results of plunge test at three flow velocities, and the relative errors are all less than 5%, verifying applicability of the established conversion relationship to the experiments.