Measurement of fish target strength is one of the core components of fisheries hydroacoustics. Research focused on offshore fish in the South China Sea has not met the needs of fisheries-resource estimations. As a typical offshore fish, little is known about the target-strength characteristics of S. sihama, a Kirchoff ray-mode approximation was used to calculate the theoretical target strength of 19 samples and the morphological parameters, calculated using a model of the fish, were obtained using an X-ray machine (SOFTEX M-100) belonging to the South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences. The changes in patterns of target strength of with tilt angle were obtained by calculating programmatically at different frequencies. The empirical formulas of target strength of with body length were established for different frequencies and the distribution function of the tilt angle was produced using a data-fitting method, and compared with the conventional expression. The results show that the various patterns of target strength of with tilt angle at 70 kHz, 120 kHz, and 200 kHz, show multiple-peak-like distributions, and the variation of target strength with the tilt angle is more sensitive at higher frequencies. As the number of crests increased, the maximum target strength corresponding to the tilt angle increased. Maximum target strength of occurs between -15° and 5° at 70 kHz, and the maximum target strength appears between -10° to 0° at 120 kHz and 200 kHz. The position of maximum target strength occurred differently at each frequency. The varying characteristics of target strength of with body length changed under different frequencies and distribution functions of the tilt angle. When the angle function is (-5°, 15°), frequency is 120 kHz, and when the angle function is (0°, 10°), frequency is 200 kHz, the empirical formula and the conventional expression curve of target strength with body length coincide, the degree of fit is higher, and the conventional S. sihama. In other cases, there are some deviations in the two formulas, and the direct fitting parameter equation is more appropriate. The study confirms that the Kirchoff ray-mode approximation can reflect the target-strength characteristics of , providing a useful reference for the study of target strength of inshore fish in the South China Sea and a scientific basis for improving the accuracy and credibility of hydroacoustic assessment of fisheries resources.