An online mobile water quality monitoring system, which is based on wireless sensor networks, is developed to solve the existing problems in the static off-line monitoring methods, such as large sampling errors, low monitoring frequency, dispersed monitoring data and lack of real-time feedback on continuous dynamic changes of water quality. The system is composed of an underwater monitor, buoy nodes, and visual software. The exterior design of the underwater monitor mimics tuna appearance, and the internal is equipped with water quality sensors (regarding temperature, pH, turbidity, conductivity, etc.) to acquire the parameters of water quality. Buoy nodes, depending on solar power, are responsible for receiving, processing, relaying, and forwarding the data from multiple underwater monitors. The underwater monitors communicate with each other and the buoy nodes through a 170 MHz wireless radio frequency module or underwater sound, while the buoy nodes communicate with shore stations by ZigBee. In addition, visual software includes PC and mobile terminals. This system overcomes the limitations of traditional monitoring based on manual sampling, achieving online monitoring of water quality at any point.