Due to the high cost for offshore operation, there are usually few number of fluid analysis tests during reserve evaluation. Analogy method are commonly used on how to determine the gas formation volume factor, specific method is to analyze the gas Z-factor and formation volume factor of the main gas reservoir, while other gas reservoir will refer to these experimental results directly, which will lead to great error when calculating the gas reserves. A new method of calculating the gas Z-factor are formed based on the relationship of gas Z-factor with temperature and pressure of the existing experimental data. The error between gas formation volume factor calculated by this binary regression method and measured value is less than 2%. Compared with the previous method, this method can get a much more accurate result while calculating the gas reserves.