To explore a gas chromatographic method to determinate molar substitution (MS) of cellulose glycerol ether (GEC) as intermediate of energetic binder，GEC was decomposed to isopropyl iodide by HI at 150 ℃ under acid condition，and the products were determined by GC-MS. The reaction mechanisms were inferred. By means of determination of isopropyl iodide in reaction products with GC，the content of dihydroxypropyloxy in GEC and MS of GEC were calculated. Results show that the MS of three GEC samples are 0.375，0.425 and 0.531 respectively，and RSD are less than 0.2%，and recovery rate are more than 96%. Compared with the ways of chemical titration and element analysis (EA)，the GC method can be used to determinate the MS of GEC and others of hydroxyalkyl cellulose ethers with a good reproducibility and high accuracy.