Abstract: The present invention provides a drainage trapping device which can prevent waste gas or poisonous gas containing bacteria from flowing backwards. The drainage trapping device comprises a water inlet pipe, a drainage trapping cartridge, a water outlet pipe, a float valve and a retaining component. The drainage trapping cartridge is provided with a water outlet which is connected to the water outlet pipe. A lower end of the drainage trapping cartridge is connected with the water inlet pipe. The float valve is disposed inside the drainage trapping cartridge and hermetically engages with a lower part of the drainage trapping cartridge when the drainage trapping cartridge is devoid of water. The retaining component is disposed at the water outlet of the drainage trapping cartridge or in the water outlet pipe, for preventing the float valve from running out.