A folding sofa bed, comprising: a bed frame comprising: 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 disposed with a backwardly disposed and downwardly inclined sliding groove having an upper end and a lower end, two hinges and two connecting bars, each of the hinges comprising a front hinge plate and a rear hinge plate, the front hinge plate connected to the seat mattress, the rear hinge plate connected to the seat back, the front hinge plate disposed with a guide bar, the guide bar slidably disposed inside the sliding groove, the two ends of the connecting bar being respectively hinged to the armrest border and the seat back, the connecting bar supporting the seat back forwardly and inclined upwardly, a seat mattress and a seat back attached to the bed frame, the seat back and the seat mattress changeable between an unfolded bed position and a folded sofa position; wherein: in the unfolded bed position, the guiding bar is situated at the upper end of the sliding groove, the seat mattress is horizontally supported by the front horizontal bar, the seat back is horizontally supported by the rear horizontal bar; and in the folded seat position, the guiding bar drops to the lower end of the sliding groove, the seat mattress is inclined and supported by the front horizontal bar, the seat back is inclined and supported by the connecting bars. a bed frame comprising: 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 disposed with a backwardly disposed and downwardly inclined sliding groove having an upper end and a lower end, two hinges and two connecting bars, each of the hinges comprising a front hinge plate and a rear hinge plate, the front hinge plate connected to the seat mattress, the rear hinge plate connected to the seat back, the front hinge plate disposed with a guide bar, the guide bar slidably disposed inside the sliding groove, the two ends of the connecting bar being respectively hinged to the armrest border and the seat back, the connecting bar supporting the seat back forwardly and inclined upwardly, a seat mattress and a seat back attached to the bed frame, the seat back and the seat mattress changeable between an unfolded bed position and a folded sofa position; wherein: in the unfolded bed position, the guiding bar is situated at the upper end of the sliding groove, the seat mattress is horizontally supported by the front horizontal bar, the seat back is horizontally supported by the rear horizontal bar; and in the folded seat position, the guiding bar drops to the lower end of the sliding groove, the seat mattress is inclined and supported by the front horizontal bar, the seat back is inclined and supported by the connecting bars. in the unfolded bed position, the guiding bar is situated at the upper end of the sliding groove, the seat mattress is horizontally supported by the front horizontal bar, the seat back is horizontally supported by the rear horizontal bar; and in the folded seat position, the guiding bar drops to the lower end of the sliding groove, the seat mattress is inclined and supported by the front horizontal bar, the seat back is inclined and supported by the connecting bars.