The hollow-core optical fiber gas laser (HOFGLAS) is emerging as a promising technology in the generation of mid-infrared radiation, which combines the advantages of both fiber and molecular gas lasers. Whilst the experimental study is of rapid development, the mechanism of lasing and explanations on emission power characteristics are yet to be discovered further. We introduce a typical four-energy level system of acetylene molecules and hence build up a rate-equation model to simulate the output power of different transitions and their relative strengths. The numerical values of slope efficiency and power ratio coincide with prior laboratory measurements. The pump threshold is numerically estimated as well.