The invention discloses a sunken pipe assembling method for making a pipeline pass through a river, and belongs to the technical field of underwater pipeline laying. The method includes the steps that first, pipes are arranged for the pipeline, the assembly procedure includes the pre-assembly process and the pontoon assembly process, the first pre-assembly pipeline and the second pre-assembly pipeline are assembled beside the bank of the river, position point information is marked on the end face of each pre-assembly pipeline, and a first river bed straight pipe segment and a second river bed straight pipe segment are assembled on a platform of a pontoon; second, pipe sinking is performed for the pipeline, the overall pipeline sinking technology is adopted in the pipeline sinking process, a first buoy is tied to a first elbow segment on the pipeline, a second buoy is tied to a second elbow segment, and by controlling a first valve, a second valve, the first buoy and the second buoy always in the center line of a pipe ditch, it is guaranteed that the pipeline accurately sinks into the pipe ditch. With the method, the whole pipeline can be accurately controlled to sink into the assigned pipe ditch, and the control accuracy is high.