Abstract: Imaging sonar can measure the length of fish images within its scanning beam. To improve the accuracy of imaging sonar's measurement of fish length, we conducted an experiment to determine the fish length by using Adaptive Resolution Imaging Sonar (ARIS). We compared and analyzed the fish length error in acoustic image based on the measured length of a trail fish and the image length at a known position of the ARIS scanning beam. The results indicate that the main influencing factor of fish image length measurement error was the angle between the fish and the sonar beam. As the angle increased, the measurement error of fish length decreased. There was no significant interactive effect of detection distance of ARIS on length measurement error of fish in the 4 M field. The error between the fish image length and the fork length was minimal, with an average error of 2.1 cm. Furthermore, the image length of fish had a good linear relationship with the total length and fork length, and the fitting degree R² of the linear modified model was 0.995 1 and 0.990 5, respectively. The study shows that fish image length based on fork measurements is more effective. Additionally, the error analysis of the angle between fish and sonar beam on image length and the sonar image length modified model also provide preliminary references for obtaining more accurate fish length information.