Salinity is one of the most important factors influencing distribution, abundance and the physiology of aquatic animals. Changes in ambient salinity are directly related to osmoregulation capacity. During acclimation to salinity, the main challenge for aquatic animals is to regulate their osmotic pressure to maintain normal life activities. As the predominant aquatic animal, crustaceans can dwell in seawater and fresh water due to their various osmoregulatory mechanisms. The two forms of osmoregulatory mechanisms are anisosmotic extracellular regulation (AER) and intracellular isosmotic regulation (IIR). AER can mainly control the osmolality of the body fluid through regulation of gills and antennal glands, while IIR maintains intracellular osmolality and maintain the balance between tissues and the hemolymph primarily by regulating amino acids (FAAs) especially non-essential amino acids (NEAA). Under excessive changes in environmental salinity, it's difficult for body fluid osmolality to be maintained at a completely stable level through AER. At that point, it's believed that IIR compensates for AER by accumulating or degrading some particular NEAA as osmolytes, effectivley moderating fluctuations in extracellular osmolality and facilitating volume readjustment. Hemolymph is directly involved in responding to salinity exposure and muscle is the major tissue for protein deposition and possibly represents the main pool of amino acids. Therefore muscle tissue and hemolymph are very important to study FAAs's involvement in osmotic pressure regulation.The experiment analyses the free amino acids content and concentration variation in muscle and hemolymph of Portunus trituberculatus at different salinities. Clarify the composition of FAAs and its function in the salinity adaptation.Enrich the FAAs in the field of salinity adaptation and provide the basis for further study of the molecular mechanism. The contents of FAAs were measured at salinities and the experiment lasted for 5 days.Determination of FAAs extraction solution of muscle and hemolymph with Hitachi 835-50 automatic amino acid analyzer.Results showed that the highest amounts of individual FAA in seawater-adapted P. trituberculatus was exhibited by Tauine, Arg, Gly, Pro and Ala. As salinity increased, the total free amino acids (TOFAA) of the hemolymph and muscle increased significantly. NEAA increased significantly when the salinity varied from 10 to 50 while Essential FAA (EAA) was not affected by external salinity change. The result showed that FAAs played an important role in salinity adaptation, especially Pro, Ala, Gly, Asp and Glu.Because Ala, Gly, Asp, Glu belong to the taste amino acids (TAA), so TAA played an important role in osmoregulation.The content of Pro increased significantly both in muscle and hemolymph, especially from 40 to 50 period. Therefore Pro played the most important role in hyperosmotic stress.