A displacement auxiliary device for an accessory of a chair is revealed. The auxiliary device is disposed on the chair and used in combination with the accessory of the chair such as an armrest, a lumbar support, a headrest, etc. The auxiliary device includes a displacement member, a fixing plate and a loading plate. The displacement member further includes two rotating members that are rotated a large angle around two rotary shafts respectively and symmetrically to be moved inward to a first position or extended outward to a second position. The accessory of the chair stays still without displacement when the two rotating members are moved inward to the first position. The accessory of the chair moves synchronously while the two rotating members are extended outward to the second position. Thereby the accessory of the chair can be adjusted to have larger displacement.