The invention discloses an electroplated diamond fretsaw production line online baume degree detection device and a detection method. The device comprises a container with a liquid inlet, a liquid outlet and a liquid overflow port. The liquid overflow port is located at a side wall close to a top of the container. The liquid inlet and the liquid outlet are arranged in a lower portion. The liquid outlet is located on a bottom of the container. A first pressure sensor and a second pressure sensor with a known height difference are arranged on an inner wall of the container between the liquid inlet and the liquid outlet, and a baume degree is calculated through a pressure difference and the height difference of the two pressure sensors. A problem of online real-time detection of the baume degrees of pre-plating, sanding and thickening sections on an electroplated diamond fretsaw production line can be solved, an automatic control system can be connected, and functions of abnormal detection value alarm, historical curve query and the like can be realized.