Effects of heat treatment on the gel properties of the body wall of sea cucumber (Apostichopus japonicus). The sensory texture of sea cucumber (Apostichopus japonicus) was dramatically affected by heat treatment. In this study, sea cucumbers were heated under different thermal conditions (HSC), and divided into five groups (HSC-80, HSC-90, HSC-100, HSC-110, and HSC-120) according to the heating temperature (from 80 to 120 °C). The changes in texture, moisture, gel structure, and biochemical parameters of the HSC were investigated. With increasing heating time (from 10 to 80 min), the hardness and gel structure changed slightly, and the water activity decreased as the proportion of T21 increased by 133.33, 55.56, and 59.09% in the HSC-80, HSC-90, and HSC-100 groups, respectively. This indicated that moderate heating conditions (below 100 °C) caused gelation of sea cucumbers in HSC-80, HSC-90, and HSC-100 groups. However, as the water activity increased, the hardness declined rapidly by 2.56 and 2.7% in the HSC-110 and HSC-120 groups, with heating time increased from 10 to 80 min. Meanwhile, free hydroxyproline and ammonia nitrogen contents increased by 81.24 and 63.16% in the HSC-110 group; and by 63.09 and 54.99% in the HSC-120 group, as the gel structure of the sea cucumbers decomposed in these two groups. These results demonstrated that, severe heat treatment (above 100 °C) destroyed the chemical bonds, triggered the disintegration of collagen fibers and the gel structure of sea cucumbers, and transformed the migration and distribution of moisture, finally causing the deterioration of the sensory texture of the sea cucumbers.