The composition and cage-occupancy of guest species in a CO2-methane mixed hydrate system are investigated by use of Raman spectroscopy. A single crystal of mixed gas hydrate shows the homogeneous mole fraction in equilibrium. The CO2 molecule is entrapped into the M-cage prior to the S-cage comparing to methane. The relative cage-occupancy of CO2 and methane in the structure-I hydrate lattice is scarcely affected by pressure condition.