The invention relates to an atmospheric heavy metal on-line detection system. The atmospheric heavy metal on-line detection system is characterized by comprising a dehumidification device, wherein sample gas enters from an inlet at the upper end of the dehumidification device, an outlet at the lower end of the dehumidification device is connected with an input end of a particulate matter concentration device through a particle size cutting device, an output end of the particulate matter concentration device is connected with an input end of an atomic thermal excitation device, the input end of the atomic thermal excitation device is also connected with an argon gas source, water cooling devices are arranged on the side walls of two sides of the atomic thermal excitation device and connected with a refrigeration cooling device through a water pump, an output end of the atomic thermal excitation device, the particulate matter concentration device and the argon gas source are respectively connected with a flow controller, a control end of each flow controller is electrically connected with a flow control system, an output end of each flow controller is connected with a main pipeline, and both the flow control system and the atomic thermal excitation device are controlled by a control terminal to work. The atmospheric heavy metal on-line detection system can be widely applied in automatic monitoring for environmental protection.