At present, the continuous radon monitor based on the electrostatic collection method has been widely used. The collection efficiency of the positively charged radon progeny in the electrostatic field of the measurement cell related to the OH - ions concentration of water vapor molecules. Using the analytical expression of equilibrium ion content of water vapor in air from literature, we obtain the analytic expression of 218 Po collection efficiency related the temperature and humidity from kinetic theory. Thus, multiple measurements can be use...