A piston assembly is composed of a piston and a piston ring, and the piston is connected with a piston rod through a thread. The utility model is characterized in that an annular groove is formed in the middle of the piston, and the cross section of the annular groove is rectangular. The piston ring is sleeved in an annular groove. The piston ring is a rubber ring. The height of the piston ring is less than the height of the rectangular groove. The piston end surface is evenly distributed with a passage communicating with the inner surface of the piston ring. The outer diameter of the piston is smaller than the inner diameter of the cylinder, and the outer diameter of the piston ring is slightly larger than the inner diameter of the cylinder. The utility model has the advantages of simple structure, reliable use, low manufacturing cost, and greatly improving the work efficiency of the air cylinder, the compression sprayer, and the piston type liquid pump. Easy to replace wearing parts.