Provided herein is a disinfection device for drainage system comprising: a housing comprising a chamber and a top end, the chamber having a body and a reducing part, the reducing part including a first opening at the bottom end of the chamber and a floating body configured to seal the first opening at the bottom end; a spiral guiding channel having an inlet and an outlet, the spiral guiding channel connected to the chamber through the outlet; an exhaust conduit having an expandable part at one end of the exhaust conduit, the exhaust conduit connecting to the top end of the housing and partially extending through the body of the chamber; and a disinfection part equipped in the interior of the disinfection device so as to remove air contaminants when air passing through the chamber from the spiral guiding channel to the exhaust conduit.