Compared to high pressure gas medium, the signal attenuation in low pressure gas is more serious with low Signal Noise Rate. So the gas flow measurement is more difficult. In order to solve the problem, a ultrasonic gas flow meter with transit time method is presented. The flow measurement model has been derived to reduce the affection of temperature and pressure on ultrasonic sensors and circuits. The method of multipulse driving and digital Automatic Gain Circuit (AGC) are proposed to obtain stable echo waves. The comparator circuit is designed and a noise filtering method is designed to stabilize the transit time measurement result. The time measurement circuit adopts chip TDC-GP22 and the microchip adopts MSP430F448. Finally, the gas flow meter is tested in the standard device. The experimental results show that the accuracy class is 1.5 level and the turndown ratio is 60:1.