In open fuel assemblies, there is a phenomenon of coolant mixing between adjacent subchannels during the flow process, which affects the flow field distribution inside the core. Therefore, the mixing characteristics of fuel assemblies are important for the safety and economy of reactors. The mixing coefficient is an important parameter that characterizes the mixing characteristics of fuel assemblies and is also an important input parameter of subchannel analysis code for safety analysis. However, due to the complexity of the mixing mechanism in the rod bundle subchannels, the mixing coefficient can only be obtained through experimental research. An experimental research on the mixing coefficient of a new type of pressurized water reactor fuel assembly has been performed. The experiment used a 5 × 5 uniform heating rod bundle with full-length and a radial power ratio of 1:3 to simulate the fuel assembly, and developed a specialized temperature measurement component to obtain the center temperature of each subchannel outlet. The research proposed an efficient method for processing data on the mixing coefficient experiment, and combined it with a subchannel analysis code to obtain the mixing coefficient of the fuel assembly. The research analyzed the relationship between the mixing coefficient and thermal parameters, and the results indicated that for a fuel assembly with specific structure, the mixing coefficient is basically not affected by thermal parameters.