Abstract: In order to monitoring on the dissolved oxygen (DO) status of different water layers in marine ranching in the real-time, online and continuous, this paper designs a multi-layer marine dissolved oxygen online monitoring system. It integrates an optical dissolved oxygen sensor developed independently and designed the chain-type DO section measurement structure. The data acquisition and control system is designed based on STM32 single-chip. And the wireless transmission module communicating with the remote control center is used to complete the real-time update and visual display of the data, which realize the expression of the remote control command to the monitoring system. The results show that the drift of DO probe can be controlled within 3%, and the whole system displayed the good stability, high reliability. It is convenient for users to check the current dissolved oxygen status of seawater and view historical data. In 2016 to 2018, the system has been applied to the east ocean Yunxi marine ranching in Yantai, Shandong. The observation data shows that DO and temperature show similar laws. The concentration of DO in the surface water is high with small variation range. The variation range of DO in the middle sea water is obviously higher than that in the surface layer and the bottom layer. And the DO in the bottom sea water shows a continuous downward trend without obvious vertical mixing. During the observation period, the bottom sea water of marine ranching appeared hypoxia for more than 18 days in 2016 and 2017, and the time series change trend of the test data is consistent with the weather change, which provide early warning and forecast for the aquaculture enterprises of marine ranching and reduce the economic loss caused by hypoxia disaster.