This study was conducted to evaluate the effects of chronic ammonia nitrogen (N) stress (concentration gradient:0 mg/L, 2 mg/L, 4 mg/L, 6 mg/L, 8 mg/L, 10 mg/L) on the feeding and activities of three types of digestive enzymes (protease, amylase, lipase) in sea cucumbers ( Selenka) of three different sizes (initial body weights:15.13±0.02 g, 28.42±0.32 g, and 60.14±0.88 g) over a 14-day period. The results obtained were as follows. (1) With an increase in ammonia-N level, the survival and growth of sea cucumbers were significantly different from those in the control group. When the ammonia-N level was higher than 2 mg/L, the final body weight and specific growth rate of sea cucumbers show a marked downward trend with an increase in ammonia concentration. As the ammonia-N level increased to 8 and 10 mg/L, all sea cucumbers of the three different sizes appeared evisceration, with skin ulceration until death. When the ammonia-N level was 10 mg/L, the survival rate of medium-sized sea cucumbers reached 81.3%, the lowest among the three sizes. (2) When the ammonia-N level was lower than 10 mg/L, the effect of ammonia-N stress on the feeding rate of sea cucumbers of all three weights was not significant (>0.05), whereas the food conversion efficiency of all the experimental groups was significantly lower than that of the control group (<0.05). (3) There were significant differences in the digestive enzyme activities of the three-sized sea cucumbers under the stress of ammonia-N. With an increase in ammonia concentration, protease activities decreased. When the ammonia-N level was 8 mg/L and 10 mg/L, the protease activities of all the experimental groups were significantly lower than those in the control group (<0.05). With an increase in ammonia concentration, the activities of lipase and amylase initially increased and then decreased, peaking at 2 mg/L. When the ammonia-N level was 6 mg/L, the amylase activity of the small-sized sea cucumbers decreased to 0.30 U/mg protein, which was significantly different from the control group (<0.05). When the ammonia-N level was from 6 mg/L to 8 mg/L, the activity of lipase in the three-sized sea cucumbers was markedly affected, being significantly lower than that in the control group (<0.05). Under the same ammonia concentration, the activities of digestive enzymes in the medium-sized sea cucumbers were significantly higher than those in the other sizes. These results indicate that ammonia-N stress can affect the feeding and digestive enzyme activities of sea cucumbers. Therefore, in the process of sea cucumber breeding, particularly in large-scale seed culture, it is preferable to control the concentration of ammonia-N in the water environment to within 4 mg/L.