A single piece water resistance door suitable for being fixed to a casing of a connector module is provided. The casing includes a first engagement part and an opening adjacent to the first engagement part. The single piece water resistance door includes a fixing part suitable for being fixed to the casing and having a first portion and a body rotatably connected to the fixing part to cover the opening. A space is between the first portion and the casing. The body includes at least one second engagement part. When the second engagement part is engaged with the first engagement part, the casing and the body press against each other, and the body covers the opening. When a force is applied to the first portion, the first portion is moved to the space and moves the body, so that the second engagement part is released from the first engagement part.