Aiming at the problem that it is difficult to determine the arrival time from the echo signal of the ultrasonic gas flowmeter and the instrument is required to have high realtime, a new signal processing method for ultrasonic gas flowmeter based on peak fitting of rising echo and a method to determine the fitting peak range are proposed. Firstly, according to the fitting peak range, several peak points are selected in the rising section of the echo signal, and they are fitted by the least squares to achieve the feature straight line. The intersection of this line and the Xaxis is regarded as a feature point to determine the arrival time of the echo signal, and then the ultrasonic propagation times are calculated to obtain the gas flow rate. The ultrasonic gas flowmeter is developed to implement the signal processing method based on peak fitting of rising echo. The calibration experiments are preformed to validate the effectiveness of the methods and the meter.