The orifice flowmeter is a kind of flow measurement element which is commonly used in the field of environmental monitoring. According to the difference of upstream inlet pressure of calibrated orifice flowmeter, calibration methods of the orifice flowmeter can be categorized into two kinds of calibration conditions, which are micro positive pressure and micro negative pressure. However, both working conditions are recommended by national standards and specifications, and the deviation of the two calibration conditions is up to 10% . The calibration deviation of the two working conditions is caused by additional differential pressure value introduced by clamping device used in positive pressure condition and linearity error introduced by the lack of available expansibility factor formula. The forms and structural parameters of clamping device which is insensitive to the upstream pressure of the calibrated orifice flowmeter are designed by the computational fluid dynamics simulation method. An available expansibility factor formula for the orifice flowmeter is calculated based on the simulation data. The achieved formula is used to carry out evaluation. The deviation of flow coefficient between the micro positive pressure and micro negative pressure calibration is less than ±0. 5% . The flow coefficient linearity of the orifice flowmeter within the calibrated flow range is also improved by about 0. 1% .