Radon is soluble in water, and the radon in drinking water is one of the sources of indoor radon. The China national standard for drinking water (GB5749-2006) indicates that the radon concentration in drinking water cannot be higher than 300pCi/L. The widely used method for measuring radon concentration in water is to use the RAD7 and the H2O accessory which provided by the Durridge Company for bubbling measurement. There are two kind sample bottles: 40ml and 250ml; the measurement steps: bubbling for 5 minutes, and measuring for 20 minutes. For the 40ml sample volume the conversion coefficient is around 25. For the 250ml sample volume the conversion coefficient is around 4. Since the price of the accessory is more than one thousand dollars, and the drying tube of the accessory is small, it needs to be replaced after each measurement which makes the operation is troublesome. We used a big drying tube to perform the radon concentration in water measurement. A new model for description the radon concentration in the gas circuit is proposed, and the conversion coefficient in any measurement condition is obtained. Any volume of sample bottle and drying tube can be used, selecting the suitable bubbling time and measuring time, the radon concentration in water can be obtained.