Abstract: The glass fiber-hollow glass microsphere/epoxy syntactic foam material with different fiber mass fractions were prepared. The effects of fiber mass fraction on mechanical properties of composite foam material were studied by three point bending test. Composite foam material specimens were put in distilled water and seawater. The influences of soak corrosion on flexural properties of specimens were studied, and the reasons were analyzed by combining with the scanning electron microscopy photos. The research reveals that the moisture rate of glass fiber-hollow glass microsphere/epoxy composites increases with the increase of fiber mass fraction, and the moisture rate of specimen in distilled water is greater than that of specimen in seawater. The flexural strength of specimens increases with the increasing fiber mass fraction, and it goes to be maximum when the fiber mass fraction is 10%, more than that of specimen without fiber by 51%, then it decreases with further adding fibers. The flexural properties of specimens are reduced by soak corrosion, and it is lower in seawater than that in distilled water. The interface layers between glass microsphere, glass fiber and epoxy matrix are damaged by soak corrosion, and it is the reason of the decrease of flexural strength of glass fiber-hollow glass microsphere/epoxy composites.