Abstract: To investigate immersion freezing (IF) effect on protein denaturation of prepared grass carp fillets (PGCFs) during frozen storage, we investigated the protein solubility, sulfhydryl of myofibrillar protein, Ca²⁺-ATPase activity, degradation of myofibril and secondary structure of myofibril. The results show that the solubility of myofibrillar protein, contents of total sulfhydryl and active sulfhydryl, activity of Ca²⁺-ATPase of PGCFs by using ICF were higher than those by using air freezing (AF). After frozen storage for 150 d, the contents of protein solubility, total sulfhydryl, active sulfhydryl and Ca²⁺-ATPase activity of PGCFs by using IF were increased by 17.51%, 14.20%, 4.86% and 28.73% than those by using AF, respectively. It is showed that IF was more advantageous to slow denaturation of protein. Moreover, the electrophoresis result show that the color of heavy chain from myosin of PGCFs by IF was deeper than that by AF, and the color of tropomyosin protein stripe was shallower than that by AF during frozen storage. Thus, IF reduced degradation of protein. The research of infrared spectrum shows that the decreased degree of α-helix content from PGCFs by using IF was less than that by using SAF. Conversely, the increase degree of β-fold, random coil and β-turn from PGCFs by using SAF were more obvious than that by using IF, which indicates that IF was more conducive to maintaining stability of secondary structure of myofibrillar protein during frozen storage. In general, IF was more beneficial to reducing the quality degradation of PGCFs due to protein denaturation.