The invention discloses an installation method of a steel approach bridge when the lifting equipment capacity is insufficient, which comprises the following steps of: anchoring a pile driving ship directly in front of a No. 1 transition pier; temporarily fixing the No. 1 steel approach bridge on the No. 1 barge; and berthing the No. 1 barge perpendicularly to the pile driving ship at the forward end of the pile driving ship through an auxiliary ship; By lifting the steel approach bridge to a certain height, the steel approach bridge is cantilevered and supported on the No. 1 barge. After the tide rises to the design elevation, the No. 1 barge is rotated 90 degrees by the auxiliary ship, so that the No. 1 steel approach bridge and the No. 1 transition pier are butted, and the steel approachbridge and the transition pier are hinged with fixed chains respectively; The second end of the steel approach bridge is lifted by a pile driver, the lighter is rotated 90 degrees to its initial position by an auxiliary ship and a winch, the steel approach bridge is lowered by a pile driver, and the steel approach bridge and the lighter are fixed by a second fixing chain. As that installation iscarry out by means of an external force, the invention is suitable for linear steel structure installation engineering under the condition that the capacity of the water hoisting equipment is insufficient.