Hydrogen fuel cell vehicles have significant advantages over traditional vehicles in energy saving and environmental protection. In the external circulation line of the fuel cell reaction device, hydrogen exists in the form of wet gas. High-precision measurement of wet gas parameters in a small diameter pipe can provide useful flow state information for monitoring hydrogen circulation state, adjusting hydrogen supply strategy and maintaining stable operation of the system. However, the sensor design and the quantification of liquid phase influence are the challenging problems in the measurement of wet gas in small pipe diameter. To measure the flow rate of wet gas under small pipe diameter, an ultrasonic-electrical dual-modality method is proposed. The gas flow was measured by ultrasonic sensor, and then the cross-section gas holdup was measured by capacitance sensor to correct the ultrasonic measurement results. A 10mm wet gas experiment platform was built, the acoustic channel arrangement of ultrasonic sensor and size of electrical sensors were simulated and optimized. The average relative error of the gas velocity measurement is 4.71%, which can measure the wet gas flow under typical flow patterns.