The present invention provides a multi-stage vacuum screening device and method suitable for online extraction of liquid metal circuit tritium. The multi-stage design reduces the flow rate of the liquid metal in a vacuum chamber and increases the residence time of the liquid metal in the vacuum chamber so as to improve the extraction efficiency of the tritium. The tritium-containing liquid lead-lithium alloy flows into a multi-stage vacuum screening device, and a hydrogen sensor is used to measure the tritium content of the tritium-containing liquid lead-lithium alloy online, then the tritium-containing liquid lead-lithium alloy flows into the multi-stage vacuum screening chamber, and the liquid lead lithium forms drops at a first-level liquid droplet nozzle array under the action of the gravity and the pressure to fall down to a next-level liquid droplet nozzle array, and forms drops through the next-level liquid droplet nozzle array under the action of the gravity until the drops fall down to the bottom portion of the multi-stage vacuum screening chamber, wherein each-level droplet nozzle allows the droplet speed to 0 so as to reduce the residence time of the droplets in the vacuum chamber, and the extracted tritium is flown to the gas holding tank for purification.