A fixing auxiliary device includes a main body, at least an extension arm and an adhesive element. The main body has a first portion in contact with a connecting portion of a circuit board and a second portion coupled to an extension plate of a metallic assembly. The at least an extension arm extends from an edge of the main body and is bent toward one side of the main body so as to clamp the extension plate of the metallic assembly. The adhesive element is used for bonding the first portion of the main body onto the connecting portion of the circuit board.