Disclosed is a flapper drain valve adjusting structure including at least two installation positions disposed at connecting positions of a flapper body and a swinging link rod, a rotating shaft with its axis perpendicular to an arrangement direction of at least two installation positions and a barrier block installed at the boundary of two adjacent installation positions to prevent the swinging link rod from moving horizontally between the adjacent installation positions, and a claw buckle installed on the swinging link rod and detachably and rotably latched onto the rotating shaft. This structure changes the conventional adjusting method and uses the fixed barrier block to separate the adjacent installation positions without the problem of worrying about the precision and structure and adopts the detachable claw buckle to provide a convenient way of replacing the claw buckle that may be damaged after a long time of use.