The characteristic analysis of the seawater hydrate enrichment area can be used as an important basis for deep-sea combustible ice exploration. In order to analyze the dissolved gases in seawater in real time, the off-axis integrating cavity output spectroscopy (ICOS) is used to detect the dissolved gases obtained by membrane separation technology. A precise spectrum analysis instrument suitable for deep-sea navigation observations is built, and theoretical analysis and experimental verification are carried out. The experimental results show that the detection range of CH4 concentration (volume ratio) is 1.073×10-8-1×10-3, the detection range of CO2 concentration (volume ratio) is 3.39×10-6-1×10-2, the measurement accuracy of CH4 isotope abundance is 1.2‰, and the measurement accuracy of CO2 isotope abundance is 1.74‰. It is shown that the system has good sensitivity and stability, and can realize continuous navigation observation in the deep sea.