Swimming crab,Portunus trituberculatus as a fashionable table delicacy is one of the most important fishing and aquaculture species of crab along the coast of China.Molting too frequently and molting attempted are commonly found during its molting process.Therefore,it is very important to investigate the molt cycle of the swimming crab in order to provide the background information for the production and management of the species.Observations of morphological changes were successfully used for the study of defining molt stages of the swimming crab.The molt cycle of the swimming crab was divided into four basic periods:postmolt,intermolt,premolt and molt.The results indicated that the postmolt period was subdivided into stage A(parts of the body flaccid),stage B(dorsal carapace edge hard,branchial and cardiac regin still soft)based on shell rigidity changes,and the premolt period was divided into substage D₀(0<R≤0.4),substage D₁(0.4<R≤0.65),substage D₂(0.65<R≤0.9),substage D₃(R≈1),substage D₄(R≈1,the old and new carapaces were separated completely)based on the ratio that was the distance between new and old cuticles to old cuticle thickness of the end of dactyl segment in the swimming leg combining with the growth of the new carapace after dissection;opening of the ecdysial sutures in the ventral surface of thoracic and the dorsal of cheliped merus was the omen that was the beginning of ecdysis.The results showed that the technique of molt cycle staging in this paper was a simple,fast,nondestructive and reliable method for the swimming crab.