In order to know the fish resource distribution and the variation of the biomass density, three acoustic surveys were conducted with a split-beam echo sounder (EY60 model, 200 kHz, Simrad) inside and outside the algae breeding zone of Baishawan in Nan'ao on Apr. 28, Dec. 2, 2015, and Apr. 19, 2016. The acoustic assessment results showed that the resource abundance density of three times inside and outside the algae breeding zone were 3.45/1.38×10⁵, 0.63/1.45×10⁵ and 3.67/2.98×10⁵ inds/n mile². Obviously, the biomass was higher in spring than in winter inside the algae breeding zone and the resource abundance density changed evidently as the seasons than outside the algae breeding zone. The highest resource abundance density was mainly in 2–6 m depth layer, 6–8 m depth layer took second place, and the upper and the bottom had the least. The biomass was affected by the depth of significant resources abundance density. Our research confirmed that during the period of algae breeding, the resource abundance density was higher inside than outside the algae breeding zone and changed evidently. It was important for the algal biology to change the biomass and distribution inside the algae breeding zone.