The sinking process of HDPE pipelines significantly affects the efficiency and success rate of underwater connection of HDPE pipelines,which is a very important link in HDPE pipelines offshore installation.We systematically study the watering sinking process of large diameter and over-length HDPE pipelines by the combination of theoretical analysis and practical investigation,first propose the method of using airbag and sacrificial rope to control the watering sinking of pipelines,and obtain the method and principle of determining the number of airbags.Furthermore,we propose the watering sinking process of large diameter and over-length HDPE pipelines under the condition of changeable water depth.The process has been applied to water intake and drainage engineering of large diameter and over-length HDPE pipelines installation for the integrated gasification combined cycle project (JIGCC) in Jazan,which can provide reference for similar projects in the future.