Abstract: In order to realize the accurate detection of coal mine gas density, it used white light extrinsic F-P (EFPI) interference fiber sensing gas technology, and it realized the survey of gas density information by adjusting the cavity length of EFPI optical fiber sensor. Through the ant algorithm for spectral peak positioning of interference spectrum in the normalization specific level, combined with the unimodal and bimodal measurement mode, it achieved cavity length demodulation.