The invention discloses a 
field device for biologically monitoring 
water quality online. The defects that tested organisms need to be fed by foods, the tested organisms need to be replaced periodically, monitoring is limited to a specific 
monitoring site and medium / long-term online biological monitoring cannot be realized in the conventional online 
water quality biological monitoring are overcome. The scientific requirements of medium / long-term biological monitoring early warning which reflects water 
toxicity change in a 
natural water body can be met. According to the device, a rule that 
hyriopsis cumingii can be subjected to hanging culture in lakes and reservoirs for ingesting 
plankton and a 
physiological phenomenon that the shell of the 
hyriopsis cumingii is tightly closed after the 
hyriopsis cumingii is stressed are fully utilized, and the shell motion of the hyriopsis cumingii is induced by using a sensor fixed on the shell of the hyriopsis cumingii, so that the biological 
toxicity monitoring of the 
water quality is performed. According to the device, a 
floating platform and a floating ball are fixed on a 
monitoring site by utilizing a 
mooring anchor, a 
signal emitter is supported by 
buoyancy supporting the 
floating platform, and the hyriopsis cumingii and the sensor are supported by utilizing 
buoyancy of the floating ball, so that the hyriopsis cumingii and the sensor are suspended in the 
water body. The shell closing response of the hyriopsis cumingii is induced by utilizing the sensor, a 
signal is transmitted to the 
signal emitter through a wire, the signal is received by utilizing a signal 
receiver arranged ashore for performing analytical 
processing, and the water quality is subjected to scientific early warning and monitoring.