Accelerated aging procedure and SIC test method have been used to determine the mechanical property of glass fiber reinforced magnesium oxychloride cement (GRMC). Hydration product, microstructure and corrosion degree of glass fibers have been analyzed by XRD and SEM in the oxychloride magnesium cement matrix. The mechanism of mechanical property degradation has been concluded based on the analysis of testing data. For ordinary GRMC sample, the main hydration product 5Mg(OH)2·MgCl2·8H2O (5·1·8) hydrolyzed to Mg(OH)2 easily in water. Mechanical property degraded because of the formation of leptophyllous Mg(OH)2, the looseness of interface region of matrix and glass fiber and corrosion of glass fiber. However, the extended mechanical performance was excellent due to the steadily exsit of 5·1·8 in high performance GRMC sample. It could be concluded that the 5·1·8 existing steadily in oxychloride magnesium cement matrix was the basilic premise for glass fiber which could not be corrupted easily.