An auxiliary power device includes a base (11) connected to a wheelchair and a central shaft (12) rotatably mounted to the base (11) about a rotating axis (LI). A guiding portion (16) is disposed on a bottom side of the base (11) and is spaced from the rotating axis (LI). A wheel bracket (21) is pivotably connected to the central shaft (12). An auxiliary wheel (25) is rotatably mounted to the wheel bracket (21). A rotary member (29) abuts the guiding portion (16). When the auxiliary wheel (25) deviates from the moving direction while moving on the ground (G), the wheel bracket (21) and the central shaft (12) together rotate about the rotating axis (L1). The rotary member (29) moves along and presses against the guiding portion (16), and the auxiliary wheel (25) is imparted with a returning force (F) to move back to the moving direction.
