The changes of cathepsin activity in myofibrils and sarcoplasm,degradation of myofibrillar and sarcoplasmic protein by SDSPAGE,myofibrillar microstructure by scanning electron microscopy (SEM) and transmission electron microscopy (TEM),instrumental texture,waterholding capacity (WHC) by drip loss and LFNMR for bighead carp fillets were measured during -20℃ storage.The results showed that the cathpsins were still active for thawed fillets stored at -20℃.The degradation of myosin,actin,troponin T and tropomyosin were observed.A 20 ku sarcoplasmic protein band which might be a proteolysis product of cathepsin L was found.The results of SEM and TEM showed that the muscle fibers were severely damaged at 112 d,which expressed by indistinguishable A band and I band in the sarcomere and almost disappeared M line.The hardness and elasticity of the fish were significantly decreased.The drip loss and T21,T22 and P22 of fillets increased,but P21 decreased significantly with the extension of frozen storage time.Overall the proteolysis of cathepsins on myofibrillar and sarcoplasmic protein during frozen storage led to the great destruction of integrity of myofibrillar microstructure and weakeness of fish texture and WHC.