Abstract: Accurate power coupler design is one of the key points to ensure high quality for RF gun used in FEL facility. In order to meet the design requirements of the RF gun, the simulation and test of coupling performance for the independent tunable RF gun applied in the HUST FEL-THz were conducted. First of all, the electromagnetic simulation software CST was used to calculate the RF gun both in frequency domain and eigenmode, and the results of these two methods are consistent with each other. The resonant frequencies are 2856.10 MHz and 2856.07 MHz respectively, while the coupling coefficients are 1.624 and 1.620. Additionally, the coupling performance of the RF gun which had been installed on the accelerator was measured by reflection coefficient method and cavity loss method respectively. The coupling coefficient of the 2nd cell is 1.78 and the resonant frequency is 2 855.77 MHz based on the former method on single point. Combined with both methods on this resonance point, the coupling coefficient is 1.785 by means of multipoint fitting. The results show that good agreements are achieved between simulations and measurements.