To address the challenges of online acoustic monitoring of behavior and accurate biomass assessment of large yellow croaker in complex environments such as net cages， this study investigated the applicability of a horizontal split-beam scientific echosounder system in an actual aquaculture setting. A scientific echosounder （Kongsberg EK80， 200 kHz） was employed to conduct standard sphere positioning accuracy experiments in a tank. Subsequently， the system was deployed at the Zhoushan Liuheng Marine Ranch to conduct continuous 24-hour horizontal monitoring of large yellow croaker in a net cage. Data regarding attitude angles （based on the net cage coordinate system）， swimming speeds， and horizontal-aspect in situ target strength （TS） were analyzed. The results indicated that the yaw angles of fish near the center of the cage exhibited high dispersion， whereas those near the cage edges were more concentrated， demonstrating a pattern of reciprocal swimming along the cage perimeter. The pitch angles followed a normal distribution， with a mean of 0.48° and a standard deviation of 13.38° （P>0.05）. The average swimming speed was highest during feeding periods （0.321 m/s）， followed by daytime （0.242 m/s）， and lowest at night （0.123 m/s）. The target strength relative to the horizontal incident beam showed periodic variations with the yaw angle； a cosine model provided the best fit （R²=0.50）， described by the equation TS = -43.41-6.99cos（2θ）. This study provides a scientific basis for the effective monitoring of behavioral indicators， facilitating the digital and intelligent monitoring of cage aquaculture and the precise acoustic assessment of stock biomass.