The paper explores that the Hynopsis Cumingii for pearl-rearing have to be hung in the optimum water layer, since the physico-chemical and biological characteristics differ within the depths of water layers in a closed lake such as Yuanjiahu Lake, Hunan Province.The pearlrearing began a hundred years ago, but no reports have been found concerning the optimization of water layers for pearl-rearing in closed lakes.The experiment have obviously shown that there is an optimum water layer of rearing pearl in the closed lake, and the optimum water layers in different waters are relatively unchangeable.The maximum phytoplankton biomass can be predicted by the algae AGP value measured.According to rhe effects of phytoplankton density on transparent degree, the following formula could be drawn out: Optimum water layer of rearing pearl (cm)=Transparent Degree (cm)×0.8