The invention discloses a device and a method for observing trace volatile organic compounds in atmosphere in an online manner, and relates to detection of volatile organic compounds in atmosphere. The device is provided with a six-way valve, a drying pipe, a mass flow controller, an air pump, a refrigerating sheet, a heating sleeve, a collecting trap, a circulating water radiating system, screen cloth, a drying pipe flow control valve, a flow control valve and a gas-phase chromatography sample injection six-way valve. Air samples can be fed into the collecting trap via the screen cloth, the air pump, the mass flow controller, the drying pipe and the six-way valve sequentially, the heating sleeve sleeves the collecting trap, the radiating sheet is arranged on the outer periphery of the heating sleeve, an inlet of the drying pipe flow control valve is externally connected with an N2 source, an outlet of the drying pipe flow control valve is connected with an inlet of the drying pipe, an inlet of the flow control valve is connected with an outlet of the gas-phase chromatography sample injection six-way valve and the N2 source, an outlet of the flow control valve is connected with the six-way valve, the gas-phase chromatography sample injection six-way valve is externally connected with a gas-phase chromatography pulse flame photometric detector, and the gas-phase chromatography pulse flame photometric detector is externally connected with an H2 source and a purified air source.