Abstract: To choose suitable polymer films for Juno experiment, the determination device and method for the radon permeability coefficient were studied. The radon permeability coefficients of nylon and polyethylene were tested by liquid scintillation counter under the steady state. Comparing tested values with literature values, the results show that the radon permeability coefficient of nylon is lower in common materials, having a significant effect on reducing radon concentration. Moreover, the main influence factors for measurement were obtained, including the simplified calculation method, the time of the system reaching stable state, environmental humidity and temperature, and the references for standardizing the determination of radon permeability coefficient of polymer films were provided.