Abstract: Disclosed is a modularized-reconfigurable long-term in-situ monitoring device and a monitoring method. The monitoring device comprises a fixed component. The fixed component is provided with a physical and chemical parameter sensor interface, a multi-channel Raman detection system, a camera system, an ocean current recording system, a control system, a power supply system, a lighting system, a floating body and an optical communication system. A releaser is arranged in the middle of the top of the fixed component. The releasing end of the releaser is connected with one end of a connecting rod penetrating through the fixed component, and the other end of the connecting rod is connected with a bottom counterbalance weight through a manual releaser. The working efficiency is improved, the operation risk is reduced, and the positioning precision is high.