A flap door mechanism with closable function includes a door member, a resilient member and a spring arm member. The door member is pivoted to a casing. The resilient member is installed on the door member and for driving the door member to rotate in a first direction. The resilient member includes a first end and a second end. The spring arm member is selectively disposed on the door member or on the casing. The spring arm member abuts against the first end. When the door member rotates in a second direction opposite to the first direction, the spring arm member is pushed to bend by the first end or is pressed to flatten by the casing, so as to reduce deformation between the first end and the second end of the resilient member.