The study of fluid inclusion in CH4—H2O system is very important to analysis of petroliferous basin, so it is necessary to quantificationally analyze fluid inclusions in CH4—H2O system. The Raman spectroscopic characteristics and molecule interaction of H2O, CH4, and CH4—H2O system were introduced in detail, including the thermodynamics properties of CH4—H2O system analyzed in this paper. The procedure to quantificationally analyze fluid inclusions in CH4—H2O system by Raman spectroscopy was carefully described. The calibration curve can be established based on methane concentration and Raman intensity ratio, which were got from synthetic fluid inclusions. It is just an available way to realize and ensure Raman spectroscopic quantificational analysis of fluid inclusion in CH4— H2O system. The salinity affects the results remarkably, and the Raman intensity ratio of methane to water decreases with adding salt when the concentration of methane is a constant in liquid phase. The concentration of methane in liquid phase increases with temperature rising in fluid inclusion as a closed system. The property and orientation of quartz do little effect on obtaining the concentration of methane in fluid inclusion. The results of experiments show that in situ Raman spectroscopy is an effective method to obtain the formation condition of methane hydrate in fluid inclusions. Based on the results of Raman spectroscopy and microthermometry, we can compute the parameters of fluid inclusion in CH4— H2O system by the equation of state.