Changeable sections and closestool siphon pipeline with changeable pipe diameters

Provided are changeable sections and a closestool siphon pipeline with changeable pipe diameters. The siphon pipeline comprises a water absorption port and a drainage port. A first section changeable pipeline is arranged on the siphon pipeline from the water absorption port to the highest point, the first section changeable pipeline is composed of different sections, and the water absorption port is provided with the first section which is in a rounded square shape; the highest point of the first section changeable pipeline is provided with a third section, and the third section is in the shape with an upper semi-circle and a square lower half bottom; the middle of the pipeline from the water absorption port to the highest point is provided with the second section, and the shape of the second section is the same as that of the first section. By means of the changeable sections and the closestool siphon pipeline with changeable pipe diameters, less water amount is required for generating the same siphon effect.