The invention discloses an underwater submerging method for large-diameter super-long HDPE pipelines. The underwater submerging method for the large-diameter super-long HDPE pipelines includes the following steps that S1, the two ends of each to-be-submerged pipe segment are each provided with a blind plate, and a plurality of gas bags are bound to each to-be-submerged pipe segment through hoisting straps, and the hoisting straps are in end-to-end connection through scarifying ropes; S2, watering sedimentation is performed; S3, the scarifying ropes are cut off in sequence, so that the to-be-installed pipe segments can sink with an S-shaped curve, and pipeline butt joint is performed after a butt joint end sinks to the butt joint height; and S4, after butt joint is completed, from the direction of the installed pipelines, the gas bags on the water are sequentially subjected to deflating, the to-be-installed pipe segments reach the bottom in a sedimentation manner, and submerging of theunderwater butt joint pipelines is completed. By means of flexibility of the pipelines, through the process that the gas bags are matched with the scarifying ropes, submerging of the pipe segments canbe completed, the underwater submerging method is especially suitable for submerging of the large-diameter HDPE pipelines, operation is easy, convenient and flexible, and the method is applicable tovarious water depths, low in requirement for ship machine equipment and small in number of needed operators.