Biogas yield represents a key parameter for evaluating the stability of fermentation process. In laboratory scale anaerobic fermentation experiments, the gas flow rate is generally low and the experimental period lasts long. In order to solve the problems of labor intensity and low efficiency caused by manual measurement, a system for online gas metering for low gas flow rate with high resolution and friendly interface was developed following the principle of liquid displacement. The system was designed based on the LabVIEW software. Various functions were achieved, including automatic calibration, automatic refilling the displaced liquid to compensate the loss from evaporation, real-time pressure and temperature compensation for volume, generating, displaying, storing accumulated gas production curve and data in real-time, etc. Finally, fermentation experiment was conducted to validate the system’s performance. Test results showed an average relative error of -2.30% and a maximum relative error of -3.03% compared with manual measurement, which indicated a high measurement accuracy of the system. Moreover, the results were stable even in the continuous long run for 80h, indicating a good performance at operational stability. In conclusion, the system can satisfy the need of real-time gas metering for laboratory scale anaerobic digesters, and it had great practical value.