Abstract: 【Objective】 In order to explore the allometric growth of the main organs of Hippocampus trimaculatus in the early developmental stage, enrich the basic biological data of the development of H. trimaculatus, and provide a reference for the construction of the artificial breeding technology of H. trimaculatus in China.【Method】 H. trimaculatus samples were collected from 0-28 days old. Used optical microscopy and experimental ecology to observe early development and measured the morphological characteristics of its main organs to construct an allometric growth model.【Result】 H. trimaculatus had obvious morphological differences from most fishes. The swimming organs were pectoral, dorsal and anal fins, without pelvic and caudal fins; the tail was elongated and had a strong clinging function; the body surface and head had prominent spines; the snout was elongated, its length accounted for about 50% of the head length. The relationship between the total length of H. trimaculatus and the age was y=0.9869x+7.9538(R²=0.9860). At the age of 0-28 days, the tail length(allometric growth index 1.1472) of H. trimaculatus showed positive allometric growth, the snout length (allometric growth index 0.9568) showed constant growth, and the head length(allometric growth index 0.8801), eye diameter(allometric growth index 0.7629), head height(allometric growth index 0.8836), caliber(45° allometric growth index 0.6803, 90° allometric growth index 0.6252), body length(allometric growth index 0.8787), body height(allometric growth index 0.8914), tail stalk height(allometric growth index 0.9428), dorsal fin length(allometric growth index 0.8483) and anal fin length(allometric growth index 0.6923) all showed negative allometric growth. There were inflection points for the head crown length and length of the pectoral fins, and the growth inflection points of the crown length were8 and 12 days in sequence, and the growth inflection points of the pectoral fin length were 16 days and 24 days in sequence.【Conclusion】 H. trimaculatus grows at an isokinetic snout to ensure its feeding caliber and predation ability. The positive allometric growth of the tail provides a strong clinging ability. The growth rate of the crown at different stages provides protection for it at certain stages. The rapid development of pectoral fins can effectively improve its predation ability. That is to say, there is allometric growth in the early growth and development of H. trimaculatus, and the main priority is the development of the climbing functional organs, followed by the feeding organs.