A folding sofa bed comprises a bed frame, a seat mattress and a seat back. The bed frame comprises two armrest borders and a front horizontal bar and a rear horizontal bar connecting the two armrest borders. A central portion of the armrest border is disposed with a long sliding groove of backwardly disposed and downwardly inclined sliding groove, two hinges and two connecting bars. Each of the hinges comprises a front hinge plate and a rear hinge plate, the front hinge plate is connected to the seat mattress, the rear hinge plate is connected to the seat back, the front hinge plate is disposed with a guide bar, the guide bar is slidably disposed inside the sliding groove, the two ends of the connecting bar are respectively hinged to the armrest border and the seat back.