Abstract: In the process of measurement and calculation of gas extraction amount with orifice flowmeter, we found that the formula for orifice flowmeter is numberous; value of each coefficient is not the same; the measured and calculated flow rate is not accurate; some formulas do not indicate the calculated amount is under state or standard state conditions . Through the analysis of the structure of the orifice flowmeter, the measurement principle, the source of the formula, we find the reasonably accurate formula and clarify the problem encountered in the work to facilitate the practical application.