The present invention is to provide a back support plate, which is integrally formed of plastic and includes an upper portion with an inner-side having a vertical curvature and a horizontal curvature that respectively conform to the curvature of human thoracic vertebrae and a horizontal curvature of the human back corresponding in position to the thoracic vertebrae, and a lower portion with an inner-side having a vertical curvature and a horizontal curvature that respectively conform to the curvature of human lumbar vertebrae and a horizontal curvature of the human back corresponding in position to the lumbar vertebrae. The two inner-sides are protrudingly provided with a plurality of protruding knobs which correspond in position to two lateral sides of the thoracic and lumbar vertebrae and have upwardly and downwardly decreasing widths, respectively, such that the joining section between the two portions extends to the two lateral sides of the human back.
