A total of 100 Threadsail Filefish (Stephanolepis cirrhifer) (length 7.00﹣14.50 cm, weight 7.96﹣136.54 g) were collected from Yellow Sea as experimental samples. We then observed and measured every sample by light microscopy and vernier caliper. The features of the front view of the head, side view of the body, second dorsal fins and scales were analyzed. We then measured the morphological indexes of the fish body, and carried out linear regression analysis. The numbers of dorsal fin rays, caudal fin rays, anal fin rays, pectoral fin rays were also counted. It was shown that the surface of the body was covered with a variety of fleece scales, with. bony scales resembling dentile. Each piece of scale consisted of a base plate and a bony bulge. Between the ventral spine and abdomen was a triangular contractile cortical structure; The fish skin was full of black spots and stripes. The other typical characteristic of Threadsail Filefish was that six main lateral lines were extended from each eye. The variation of total length/standard length was minimum and the difference between female and male was the lowest, and the correlation equation was SL = 0.8239TL﹣0.0471 (R2 = 0.9763), which had the best linear correlation coefficient. The calculation of head length/proboscis length and head length/eye diameter had the lowest linear correlation coefficient, and the variation and difference between female and male were the highest. The general correlation equation of standard length and weight was W = 2.3743e0.2803SL (R2 = 0.9577), and it was W = 2.5406e0.2737SL (R2 = 0.9638) for male ones, W = 2.0328e0.2954SL (R2 = 0.9490) for female ones. The first dorsal fin consisted of two dorsal fin spines; the number of second dorsal fin was 30﹣34; The number of anal fins was 29﹣34; the number of ectoral fins was 12; the number of tail fins was 13; the pelvic fin degenerated into pelvic fin spines. The external difference between the female and male ones was the length of the second root of the second dorsal fin. The length of the second fin in males was about 1/6 to 1/5 of that in females. It appears that the distribution of this species has been extended to the yellow sea.