Based on the background of a reducing subsea pipeline in a gas field located in the South China Sea， a multi-diameter pig with a reducing rate of 59% is designed， and its passing performance， wear resistance and dewatering performance are analyzed. A test pipeline is designed to simulate the actual reducing subsea pipeline system. Based on the test pipeline， using fresh water as the driving medium， the passing performance of the multi-diameter pig in the pipeline with a reducing rate of 59% is verified. The multi-diameter pig has a uniform fold shape on the sealing plate in the 20.32cm pipeline， and its operating pressure is up to 4×10⁵Pa （gauge pressure）， which meets the actual use requirements. Using compressed air as the driving medium， the dewatering performance of the multi-diameter pig is studied. The operating pressure of the multi-diameter pig running from an 20.32cm pipeline to a 30.48cm pipeline is higher than that of the pig running from a 30.48cm pipeline to an 20.32cm pipeline， but the dewatering performance in the former case is better than that in the latter case. The multi-diameter pig after simulation test will be applied to the dewatering of a subsea reducing pipeline in a gas field in the South China Sea.