This paper presents an underwater environment monitoring system, which is designed for deployment with a meteorological mast in the offshore wind farm. The factors of safety, reliable, and extendable capacity are the major design concept. First of all, the demonstration project of the offshore wind farms at Taiwan is going to build three pilot wind farms and meteorological masts. The measurement of ocean environment is included sea surface wave, sea current, sea surface level, sea temperature and salinity. Thus, the instruments of wave gauge, acoustic Doppler current profiler (ADCP), and conductivity-temperature-depth (CTD) are required for the demonstration project. Moreover, the data collection of environmental impact assessment needs underwater noise meters and ocean bottom seismometer, which are also included in the underwater environment monitoring system. Based on the measurement quality and safety, a seabed mounted frame is used for protection all instruments. The overall weight is about 100 kg in air and about 60 kg in water. The underwater environment monitoring system has an underwater junction box that is a node to integrate for power supply and data transmitting of five instruments. The electric power and digital signals are transmitted by a hybrid cable in 150 m long within four pins copper conductors and two optical-fiber cables. The power loss rate in the copper conductors is about 5.025 ohm/km with the power supply of 220 VAC and 0.55 A. The power convertor in the junction box is AC-to-DC, which has input power of 100-240 VAC and output power of 12/48 VDC. The optical-fiber cable transmits the digital data via single-mode with a bandwidth over 156 Mbps, which is required by the underwater noise data with bandwidths from 80 kbps to 2.4 Mbps. The field testing had two stages, which were in a water tank and in the harbor over ten days. The preliminary testing tasks in a water tank were the correct rate of data collection and reliability of system opera- ion. The sea trial task was deployed the underwater environment monitoring system in a water depth of 8-9 m at Kaohsiung Harbor. The results had reported the system reliability, the ocean environmental states, and the underwater noises in Kaohsiung Harbor. Moreover, the surface of the stainless steel frame and the aluminum alloy housing were weakened due to the corrosion and biofouling, which indicated more robust materials and anti-corrosion coating are demanded in the future.