Abstract: RF power coupler is one of the key components in China Spallation Neutron Source (CSNS) Drift Tube Linac (DTL), its coupling coefficient is usually obtained by changing the dimensions of coupling hole mechanically. In order to make sure that the coupling coefficient reaches the designed value, a cold model is manufactured for the coupling coefficient adjustments. During the measurement of the cold model, it is found that the coupling coefficient cannot reach the target value by changing the hole diameter only. Therefore, a new simulation model with good accuracy in comparison with measurement results is proposed for analysis, and the theory used in Proton Engineering Frontier Project (PEFP) and the main parameters affecting the coupling coefficient are analyzed accordingly. Finally, the coupling coefficient is adjusted to the target value, and the low power test results of copper model show a good agreement with the simulation results.