Abstract: This paper studied the occurrence time and physiological characteristics of slow growth seahorse. Comparisons of the growth, the ratio of RNA/DNA, digestive enzymes and related immune enzymes activities in normal growth seahorses Hippocampus erectus(8.57±0.61) cm, (1.51±0.38) g and slow growth seahorse (6.68±0.35) cm, (0.81±0.14) g were studied in a cultural experiment for 110 d. The results showed that the average daily gain, specific growth rate, condition factor and viscera index in the normal growth seahorses were significantly higher than those of the slow growth seahorse, respectively. The normal growth seahorses grew faster than the slow growth seahorses after the body length was 5.03 cm. The amylase, protease and lipase activities in normal growth seahorses were in the same tendency as those of the slow growth seahorses at different reaction temperatures (5 °C, 15 °C, 25 °C, 30 °C, 35 °C, 45 °C and 55 °C) and pH (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0). However, the digestive enzymes activities of the former were higher than those of the latter, except amylase activity at 35-55 °C and protease activity at 5–15 °C. The activities of ACP, AKP, T-AOC and SOD in normal growth seahorses were 12.83%, 48.21%, 44.71%and 65.75% higher than those of the slow growth seahorses, respectively. The MDA of the normal growth seahorse was only 61.09% of that in the slow growth seahorse, and the ratio of RNA/DNA in the normal growth seahorse was 23.75% higher than that of the slow growth seahorse. The growth, digestion and immunity of normal growth seahorses were obviously better than those of slow-growing seahorses when their body length exceeded 5.03 cm.