Abstract: Silver carps (Hypophthalmichthys molitrix) and bighead carps (Aristichthys nobilis) are often the most dominant fish species in Chinese lakes and reservoirs, where they are often stock enhanced.In order to evaluate more accurately the fish resources of lakes and reservoirs such as Qiandao Lake based on the hydroacoustics, the target strength of silver carps and bighead carps was measured using the Kirchhoff Approximation Model. A total of 24 silver carps and bighead carps were collected as samples in November and December 2021. The images were obtained by digital X-ray fluoroscopy system (SONIALVISION C200). Through software programming, the change images of target intensity of silver carps and bighead carps with fish body inclination under three common fishery acoustic investigation frequencies were obtained, and then the fitting equation of the relationship between target strength and body length was calculated. When the probability density function of attitude inclination is (−5°, 15°), the fitting equations of target strength and body length of silver carps at 70, 120 and 200 kHz are TS=20lgL−68.38, TS=20lgL−67.35 and TS=20lgL−65.93 respectively; the fitting equations of target strength and body length of bighead carps at 70, 120 and 200 kHz are TS=20lgL−69.46, TS=20lgL−67.06 and TS=20lgL−65.69, respectively. The obtained formula is in good agreement with the results measured by other scholars using experimental methods, compared with the traditional empirical formula of swim bladder fish, it greatly improves the accuracy of the evaluation results. Different from the previous research records, during the experiment, it was found that only the young fishes with shorter body length had two swim bladders. It was speculated that after the fish reached a certain body length or age, the second swim bladder degenerated. The results of this study can provide more accurate evaluation results for the information of fish resource reserves or developed degree of lakes and reservoirs such as Qiandao Lake, and make more reasonable planning for fishery production. At the same time, it’s hoped that it can be of reference significance to the research on the target strength of other fish.