Abstract: We observed and drawn the external morphology of 131 Thamnaconus septentrionalis collected from the Yellow Sea, and counted the number of dorsal fin rays, caudal fin rays, anal fin rays as well as pectoral fin rays, so as to conduct a regrassion analysis of their morphological features. Besides, the chromosome specimens were made by hot titration method, and the karyotype was analyzed. The results are: 1) when alive, the head and back of T. septentrionalis' body were in pewter and the abdomen was in whitish turquoise; after death, the color turned dark green. The fins' remaining turquoise was an obvious feature of its external morphology. 2) The body length was 1.5−3.3 times and 3.1−4.9 times as long as the body height and the head length; the head length was 1.1−1.6 times as long as the proboscis length. 3) The variation of total length/standard length was minimum with the smallest difference between female and male and had the best correlation coefficient (R²=0.96). The values of caudal length/caudal depth, standard length/caudal length, head length/eye diameter and head length/eye interal had the lowest correlation coefficient, and the difference between female and male was obvious. 4) The fin type was: dorsal fin II, 36−39; anal fin 34−37; tail fin 14−15, caudal fin 1+10+1, and the pelvic fin degenerated into pelvic fin spines. 5) The body was covered with a variety of fleece scales consisting of a base plate and a bony bulge, and the lateral lines were composed of trunk line and infraorbital line. 6) T. septentrionalis had 20 pairs of chromosomes (2n=40) and the karyotype formula was 2n=40t. All of them were telomeres without heteromorphic sex chromosomes.