Abstract: The target strength (TS) of fish and its relationship with body length is one of the core contents of research and application in fishery acoustic technology. The current study measured target strengths of two widespread commercial cyprinid fish species, Cyprinus carpio and Aristichthys nobilis with hydroacoustics experiment with tethering method and Simrad EY60 echosounder (120 kHz) under swimming status (26 individuals of C. carpio and 26 of A. nobilis) in an undisturbed cage in the Geheyan Reservoir (Yichang City, Hubei Province) from 1 May — 20 May, 2017. The total length of C. carpio and A. nobilis were from 19.9 to 29.6 cm and 49.2 to 74.2 cm, respectively. The body weight of them were from 175 to 461 g and 1200 to 4700 g, and their TS were from –59.35 to –45.20 dB and –34.22 to –17.49 dB, respectively. The regression relationship between target strength (TS, dB) and total length (TL, cm) for the two species were TS = 29.84×lgTL – 95.23 (R² = 0.74, P < 0.01) for C. carpio, and TS = 35.88×lgTL – 90.33 (R² = 0.83, P < 0.01) for A. nobilis. The TS-TL equation of A. nobilis was used to assess the total fish biomass of a small enclosed water body (Lake Yanlong) to 89.36 ton (i.e. 40.1 g/m²), which is close to the real total catch (73.61 tons) of this area, supporting the rationality of in method to evaluate fish target strengths and their relationship with body length. Exploring target strength for the two most common fish should benefit the development of hydroacoustics techniques in China’s inland waters.