Control of the gas used and doing so in a quantitative way is essential to improve the accuracy of a gas fuel system. However, this process will be affected by the fluctuations in the gas injection volume, resulting from the characteristics of the gas compression used. A novel method is proposed by the authors to offer a solution to achieve better control of the molarity of the gas used. Here, a max error of 0.715 % under the proposed control method for the gas molarity was verified by using a gravimetric analysis method. The method proposed was designed to be applied to an untethered fuel system and a 120 g hydrogen fuel system was built, where the maximum repetitive error of the output work driven by the gas combustion reached 3.55 %, which demonstrated well the controllability of the fuel system. The molarity-based control method shows advantages of high-precision, untethered use and extensibility.