Embryonic development of the Chinese mitten crab, Eriocheir sinensis, is completed in estuaries where they are exposed to frequent and abrupt changes in environmental salinity. Biochemical composition (carbohydrates, proteins and lipids) content and digestive enzyme (amylase, lipase, pepsin and trypsin) activity were determined in three developmental stages (gastrula, eyespot and pre-hatching stage) of E. sinensis eggs under hyper- or hypo-osmotic stress. Increases in pepsin and trypsin activity (P<0.05) and decreases in protein (P<0.05), lipid content (P>0.05) and lipase activity (P<0.05) in the eggs of the control group were noticed during embryonic development. Total carbohydrate content and amylase activity increased at the eyespot stage (P<0.05), whereas they reduced during the pre-hatching stages (P<0.05). The protein content during all three developmental stages decreased under hyper-osmotic stress while it increased under hypo-osmotic stress (P<0.05). Total lipid content was unaffected by hyper-osmotic stress at any developmental stage, whereas it reduced significantly at the pre-hatching stage under hypo-osmotic stress (P<0.05). Total carbohydrate content did not show any significant variation after osmotic stress. Hyper-osmotic stress reduced enzyme activity at the three developmental stages; however, hypo-osmotic stress enhanced it, with the exception of reducing amylase activity in the eyespot stage (P<0.05) and trypsin activity in the pre-hatching stage under both hyper- and hypo-osmotic stress (P>0.05). With the exception of lipase, the effect of salinity changes on digestive enzymes was only statistically significant during the gastrula stage (P<0.05). We suggest that proteins and lipids are important structural materials of embryonic development and provide energy for this process. Osmotic stress changed the activity of digestive enzymes, which further affects the use of yolk substances, and consequently affects embryonic development. Extremely high or low salinity in estuaries potentially affects the embryonic development of E. sinensis, especially under salt-water encroachment.