To provide a water cutoff processing method and a water cutoff processing structure of a wire harness, in which high water cutoff effects can be ensured easily.

A water cutoff processing method includes a winding step for winding a tape-like water cutoff material 22 at a predetermined position of each cable 12 constituting a bundle of cables 12 of a wire harness 11, a fusing step for fusing the water cutoff materials 22 of the cables 12 to each other to provide an integrated water cutoff processed part 21, and an inserting step for inserting the bundle of cables 12 through a wire harness perforated part formed in a grommet to dispose the water cutoff processed part 21 in the wire harness perforated part. In the winding step, the water cutoff material 22 is used with which self-fusing materials 24 are provided on front and rear sides of a sheet 23.