An auto-adjustable directional drilling apparatus comprises: a drive-shaft housing; a drill collar coupled to the drive-shaft housing; a drive shaft passing through the drive-shaft housing and the drill collar; an active stabilizer fixed to the drive-shaft housing and movably coupled to the drill collar; a sliding assembly comprising a base support fixed to the drill collar and a sliding base coupled to the drive-shaft housing, wherein the base support defines a slide way and the sliding base is slidably disposed in the slide way; and an actuating module coupled to the sliding base to drive the sliding base to slide along the slide way. An auto-adjustable directional drilling method is also disclosed.