Octogen HMX is separated from nitrocellulose by acetonitrile dissolved water separated out in composite modified double-base propellant and determined by HPLC on C18 alkyl-bond column (4.6 mm×200 mm, 10 μm) ,using a mixture of acetonitrile and water with the ratio of 46 to 54 by volume as the mobile phase,at a flow-rate of 1.0 mL·min_－1 ,UＶ-detection at 254 nm for the final determination. Tests for relative standard deviation and recovery are made: 0.82% for relative standard deviation′s and 99.62% for recovery are obtained for octogen at the content of 21% by quality in propellant .