The invention discloses an in-situ online heavy-metal monitoring system device which comprises an in-situ online heavy-metal monitoring tank, a PC (personal computer) and a scale removal tank. The scale removal tank is inputted into to-be-detected industrial wastewater, signal conditioning modules and data acquisition modules are arranged in the in-situ online heavy-metal monitoring tank, the scale removal tank is divided into an upper layer and a lower layer by a PVC (polyvinyl chloride) partition plate with a plurality of water inlets, sensor insertion holes are formed in the center and four corners of the top surface of the upper layer, temperature sensor insertion holes are further formed in the top surface of the upper layer, a plurality of all-solid-state heavy-metal sensors, a reference electrode and a pH (potential of hydrogen) electrode are arranged in the sensor insertion holes, a temperature sensor is inserted in each temperature sensor insertion hole, and digital signals acquired by the all-solid-state heavy-metal sensors can be transmitted to the data acquisition modules by the signal conditioning modules. The in-situ online heavy-metal monitoring system device has the advantages that the concentration of heavy-metal ions in the industrial wastewater can be monitored in real time, scale removal treatment can be periodically carried out on the sensors by the aid of ultrasonic principles, and accordingly the ion concentration monitoring accuracy can be guaranteed.