The invention discloses a stay cable replacement method based on unstressed cable length. The method is suitable for a cable stayed bridge which adopts a geometric control method for controlling construction stages and adopts parallel steel wires as stay cables. The method comprises the following steps: measuring and calculating the stressed length of a stay cable according to the magnitude of the cable force of the replaced stay cable in the current state; determining the unstressed length and on-site pull-out length corresponding to the stay cable in the current state; and controlling the final on-site pull-out length of the stay cable to realize the replacement of the stay cable. The method disclosed by the invention can be used instead of a traditional cable replacement method based on tensioning force, the tensioning force of the stay cable does not need to be controlled but only the on-site pull-out length of the stay cable needs to be controlled in the process of on-site replacement of the stay cable, and the replacement of the stay cable under uninterrupted traffic conditions can be realized.