In this paper, two optical fiber gas sensors were successfully developed, and the detection of formic acid gas was realized at room temperature. The limit of detection for the chitosan material modified probe for formic acid can reach 1 ppm. The combination of molybdenum disulfide (MoS2) nanosheets and chitosan effectively improves sensor response and recovery time. The gas-sensitivity characteristics of the two sensors have been studied, compared and discussed respectively, and the good selectivity and repeatability have been verified by experiments. By combining the gas-sensitive material with the optical fiber structure, this paper verifies and broadens the method for enhancing the sensitivity of the formic acid gas sensor and the realization of concentration detection at room temperature. Because it breaks through the limitation of high-temperature catalytic conditions, the probe is expected to be used for formic acid gas detection, and can be extended to the room temperature detection of other gases under harsh environmental conditions such as strong electromagnetic interference.