A flat drill bit includes a stem body, a cut flute part provided axially from an end of the stem body and being generally flat-shaped, and a threaded guiding part provided axially from an end of the cut flute part. The cut flute part includes two longitudinal wings which are configured to be rotationally symmetrical to each other about a longitudinal central axis (O) of the flat drill bit. Each longitudinal wing is formed with a cutting edge at the end of the cut flute part from which the threaded guiding part extends. A rotating trajectory profile caused by the cutting edge when the flat drill bit rotates is inclined towards the stem body and is at an angle (a) relative to the longitudinal central axis (O), and the angle (a) is in a range between 84° and 86°.