The invention discloses an early warning device for biomass with comprehensive toxicity in water, which comprises a water distribution area, a detection area and a control area. The water distribution area consists of a sampling tank, a clear water tank, a three-way valve, a clear water valve, a sample feeding pump, a clear water pump, a culture solution and concentrated solution storage tank, trace elements, a vitamin concentrated solution storage tank, a pH (potential of hydrogen) buffer solution storage tank, a mixing and deoxidizing tank, a magnetic stirrer, a liquid adding control valve and an emergency water sample draining pipeline; the detection area consists of a microbial fuel cell detecting unit, a sample feeding and water distributing valve, a water distributing tube, a cleaning control valve, a microbial fuel cell water discharge pipe, a depositing tank, a microorganism backwash pump, a drain pipeline and a microorganism removing pipeline; and the control area comprises a monitoring control and warning system and a display screen. By the aid of the early warning device, online monitoring and early warning for comprehensive toxicity of the water can be realized, and the early warning device has the advantages of long service life, high interference resistance and sensitivity, wide monitoring range, simplicity in daily management, low running expense and the like, and is applicable to monitoring emergency toxicity of water in conventional and emergency modes.