Abstract: Field calibration problems existed in car-mounted piston-type liquid prover of crude oil flowmeter (Roots flowmeter, duplex rotor flowmeter and scratch flowmeter) are presented. Influencing factors of flowmeters' quality, running condition, calibration process and environment on calibrating procedure of the flowmeters are analyzed. According to analysis results, countermeasures to reduce calibration error in oil metering are laid down, which can effectively raise efficiency and quality in the calibration of crude oil flowmeter and safeguard economic interests between two sides of oil custody transferring.