The first reason is that continuous rainfall leads to low salinity of the pool water and insufficient dissolved oxygen at the bottom of the pool. In the high temperature period, because of continuous rainfall, the salt content on the surface of the pond is relatively low, and the dissolved oxygen content at the bottom of the pond is less, which is the main reason for the lack of dissolved oxygen at the bottom of the pond. Toshiro Maruyama once said in a paper published in Aquaculture that through investigation and analysis, due to the influence of fresh water flow, the formation of low salinity water layer on the surface played a "blocking" role in the transfer of dissolved oxygen. Therefore, the surface water with high dissolved oxygen is not easy to reach the bottom. Especially at night or before dawn, plankton moves vertically downward, settles on the bottom and dies. The disintegration of plankton and the self-consumption of oxygen in sediments make the dissolved oxygen value in the bottom layer decrease continuously. According to our water quality monitoring, if there is heavy rainfall in the pond without timely drainage measures, the dissolved oxygen in the upper layer can reach 7 ~ 8 mg/L, while the dissolved oxygen in the bottom layer of the pond is only1.4 ~ 3.4 mg/L. This situation is more obvious in summer, and the lack of dissolved oxygen in the bottom layer is further aggravated, thus forming a stable anoxic layer. NH4+, NH3, H2S, HS- and CO2(HCO3-) are constantly produced in the sea cucumber culture pond, which easily leads to the death of cultured sea cucumbers due to lack of oxygen.