The real process of digital measure of radon is simulated by flow through radon gas source. The experimental study on the instrument standardization and each procedure of digital measure of radon is made. It was found that count rate reduces while gas flowing rate increases. This fact can be explained by the lasting time of gas in the scintillation chamber. Some conclusions are drawn: instrument of the digital measure of radon must be standardized and only standardized by flow through radon gas source; gas flux of entering the scintillation chamber should be controlled or measured in the process of digital measure of radon. In addition, the radio contamination in scintillation chamber is analyzed.