A waterproof door assembly of an electronic device having an opening with an annulus wall includes a fastening element formed on the electronic device, a waterproof door having a door panel pivotally coupled to the electronic device and a replaceable ring, and a switch flipping element. A bump and an engaging portion are extended from opposite sides of the door panel respectively. The replaceable ring is sheathed on the periphery of the bump; the switch flipping element slides on the door panel. The switch flipping element has a snapping portion and a fastening portion for performing a multi-stage position adjustment to latch the fastening portion to the corresponding fastening element. The bump is inserted into the opening, and the replaceable ring is clamped between the annulus wall and the bump to maintain high sealing performance and long service life of the waterproof door.