The invention provides a suspension bridge stiffening girder section installation method. Two cable-crossing cranes are fixed to a main cable, and extension bars are installed on a first stiffening girder section; the first stiffening girder section is in place; lifting ropes of the cable-crossing cranes are connected with the extension bars, the first stiffening girder section is lifted to the preset position, and lifting rods are installed; the extension bars are detached; the two cable-crossing cranes are moved forwards and backwards at the distance of the length of one stiffening girder section correspondingly and are fixed; two cantilever cranes are arranged on the top face of the first stiffening girder section; extension bars are installed on a second stiffening girder section and athird stiffening girder section; the second stiffening girder section and the third stiffening girder section are in place, the lifting ropes of the cable-crossing cranes are connected with the extension bars, and lifting ropes of the cantilever cranes are connected with the top faces of the second stiffening girder section and the third stiffening girder section; the second stiffening girder section and the third stiffening girder section are lifted to the preset positions, and the lifting rods are installed; the extension bars are detached; and following stiffening girder sections are installed sequentially until all the stiffening girder sections are completely installed. The method is suitable for the working condition that the mid-span stiffening girder sections are higher than the main cable.