The invention provides a diameter-variable bent pipe. The diameter-variable bent pipe comprises a first straight pipe segment, a bent pipe segment, a diameter-variable segment and a second straight pipe segment which are sequentially connected, and the first straight pipe segment is different from the second straight pipe segment in outer diameter; and all the pipe segments of the diameter-variable bent pipe are formed integrally. Correspondingly, a pipeline arrangement connection method utilizing the diameter-variable bent pipe is provided. The steering function of a pipeline is achieved while diameter-variable connection among pipelines with different outer diameters, equipment and instruments is achieved, the arrangement flexibility of nuclear power pipelines is improved effectively, the pipeline arrangement space is saved, the length of the pipelines is decreased, and the cost is lowered; and meanwhile, the number of weld seams at the diameter-variable segment of the pipeline is decreased, pipeline stress distribution is improved, the seismic performance of the pipelines is improved, the pipeline on-site prefabricated welding workload is decreased, and the pipeline on-site mounting efficiency is improved.