Abstract: In order to reveal the bone morphology of Euthynnus affinis, we studied its X-ray photography. The fish consisted of 20 trunk vertebrae and 18 tail vertebrae. The 4^th–30^th vertebrae connected with 27 spinal spines. The 16^th–28^th vertebrae extended downward and connected to each other to form a special bone structure. The upper and lower sides of the 35^th–37^th vertebrae extended obliquely to the rear to form three supraccaudal bones and three lower tail bones. The back half of the 38^th vertebra evolved into a whole tailbone. The shoulder girdle was located at the 5^th–6^th vertebrae. The girdle bone was located below the 6^th spine. The 1^st dorsal fin was located above the 6^th–16^th vertebrae. The 2^nd dorsal fin was located above the 21^st–24^th vertebrae. The anal fin was located below the 23^rd–26^th vertebrae. There were 7–8 small fins behind the dorsal fin, and the branch fin bone extended between the two spinal spines. There were 6–7 small fins behind the anal fin, and the fin bones extended between the two spines.The results show that the X-ray method can observe the bones of E. affinis clearly, but the head bones are not clear because of more overlapping; the X-ray method can achieve harmless fish bone research; in order to adapt to high-speed swimming in the sea, E. affinis has a unique bone structure. The study of bones of E. affinis enriches the research of tuna in the South China Sea, providing references for its classification and evolution, as well as providing ideas for the harmless study of rare fish.