The invention relates to a mounting box fixing piece for electrical accessories. The mounting box fixing piece for the electrical accessories is characterized by comprising a supporting piece and a wedge-shaped block; the supporting piece comprises a connecting plate; two ends of the connecting plate are respectively provided with a bending part which is bent towards the same side; the end part ofeach bending part is provided with a supporting plate; the end part of each supporting plate is provided with sawteeth; and the middle part of each connecting plate is provided with a round hole. Theside section of the wedge-shaped block is trapezoidal, a threaded hole is formed in the wedge-shaped block, a supporting piece is made of a material with certain strength and elasticity, and the wedge-shaped block is pulled through a bolt, so that the arch-shaped supporting piece expands and deforms up and down and is fixed in the mounting box. Meanwhile, the wedge-shaped block also plays a roleof a nut, so that the bolt and the cover plate are firmly fixed, and the mounting structure has the advantages of simplicity and firmness in mounting and reusability.