Abstract: An improved sofa bed structure if provided which comprises a base frame, a back support frame and a rear back support frame that are sequentially assembled using pin connections to form an integral bedstead. Each frame is equipped with a net frame to form a bed board structure. The whole bedstead is fastened by the interlocking of a base frame to a fixed base. The bedstead functions as a fully opened bed board structure or a folded sofa structure. When the whole structure is folded as a sofa, the fixed foot bars of the rear back support frame are received in saddles of the fixed base. The end portion of the rear back support frame and foot portions of the fixed base are fully in contact with one another to achieve a transversal interlock effect and to strengthen the structural stability of the whole sofa base.