Lithium iron phosphate batteries have been widely used in energy storage system because of the long service life and environmental protection. However,in recent years,there are many safety accidents in the energy storage power station,which threaten the stable operation of the power grid. In the process of thermal runaway,lithium iron phosphate batteries will produce combustible gases such as C₂H₂ and C₂H₆,which is an important cause of combustion,explosion and other disasters. Therefore,real-time monitoring of combustible gases such as C₂H₂ and C₂H₆ in the prefabricated cabin can ensure the safe and stable operation of the battery. In this paper,a fiber loop ring-down spectroscopy (FLRDS) technology based on open optical paths is proposed,which can realize on-line monitoring of trace gas in open space. The optical loss of gradient index (GRIN) lens is analyzed theoretically and experimentally. An open optical path FLRDS gas detection system with an insertion loss of 0.95 dB is established. According to the infrared spectral characteristics of C₂H₂ and C₂H₆,the performance test of laser source is carried out. The gas concentration detection methods of C₂H₂ and C₂H₆ are studied by using the simulated open space. The results show that the system has good stability,and the standard deviation S of the measured signal under N₂ background is only 0.156% of the average value. There is a good linear relationship between ring-down time and concentration. R² of C₂H₂ is 0.998 32,R² of C₂H₆ is 0.994 72. The results of the inversion calculation show that the maximum relative errors of C₂H₂ and C₂H₆ are 3.215% and 4.72%. The maximum absolute errors are 16.86 μL/L and 12.74 μL/L. The research proves that the method has good measurement accuracy.