In order to analyze space-domain passive fiber cavity ring-down acetylene gas volume fraction detection system more accurately, a theoretical model of gas volume fraction sensing system considering system noise was established in this paper. The performance of space-domain passive fiber cavity ring-down acetylene gas volume fraction detection system was simulated and discussed by using this model. Simulation results show that, by using the space-domain passive fiber cavity ring-down sensing technology, the acetylene volume fraction can be monitored by measuring the ring-down distance and the sensitivity of 56.226 km-1 is achieved, the corresponding stability and detection limit can reach 0.47% and 260.577×10-6, which can be further improved by reducing the inherent cavity loss of the fiber cavity. This research has theoretical significance for the optimization design of volume fraction sensing system.