The content of calcium in the gill and carapace of juvenile Eriocheir sinensis during the molting cycle was determined, and the histological structure of gill and carapace in different molting stages was observed by paraffin section and electron microscope. The results showed that the content of calcium in gill and carapace was the lowest in the ecdysis stage. The results of microscopic observation showed that the inner and outer cuticles at the end of gill lobes were separated, and the lumen of marginal vessel was not completely recovered. The cavity of marginal vessel returns to normal in the postmolt stage. The results of ultrastructural observation showed that in the ecdysis stage, the content in the cavity of gills was less, the staining was shallower, and the number of mitochondria and microvilli was large. In the postmolt stage, the contents in the cavity of gills increased, the staining deepened, and the number of mitochondria and microvilli was larger. The carapace was the thinnest in the ecdysis stage, and only had two layers of structure, the epicuticle and the exocuticle. The endocuticle did not form until the postmolt stage. In conclusion, the calcium content and tissue structure of Eriocheir sinensis are closely related to the molting stages.