The invention provides a mounting method of a base-free ship large-size gantry frame. According to the method, a thickened deck serves as a base of the large-size gantry frame, and the large-size gantry frame is successfully mounted on a ship deck through precision control over the thickened deck and by adopting a pre-position lineation mounting mode. The whole method is detailed in step, clear inthought, high in operability, high in safety and low in error rate.