In order to study the quality changes of grass carp during postmortem at room temperature (25℃), the changes in texture, pH, lactic acid, free amino acids, and ATP-related compounds in the dorsal meat of grass carp were measured using texture analyzer, pH meter, lactic acid kit, automatic amino acid analyzer, and high-performance liquid chromatography method in the postmortem process within 30 h. The results showed that hardness and chewiness of the grass carp reached its maximum at 23 h and then rapidly decreased, while springiness, cohesiveness, and resilience of grass carp decreased with the extension of storage time. pH showed a trend of decreasing first and then increasing, and it reached its maximum at 23 h. The content of lactic acid increased initially and then decreased, which reached its minimum value at 23 h. The total amount of fresh sweet amino acids gradually decreased during the postmortem process, while bitter amino acids were just the opposite, which increased during the postmortem process. The content of ATP increased significantly within 10 h, while the content of IMP with umami taste decreased rapidly1. Furthermore, the content of HxR and Hx with bitter taste increased gradually during the storage process. The K value increased during the postmortem process. The change in K value indicated that grass carp could maintain high freshness after storage for 10 h at room temperature, and it reached 68.99% at 27 h, which showed that the fish had spoiled at that time. Therefore, it should be processed as soon as possible, ideally within 10 hours after the death of grass carp at room temperature. This study will enrich the basic theory of postmortem variation of fish and provide support for the quality change of aquatic products.