The invention relates to a diameter-variable cylindrical air pressure spring. The current air pressure spring has the shortcomings of complex structure, insufficient deformation stroke and poor ventilation effect. The diameter-variable cylindrical air pressure spring comprises a spring body, the spring body is formed by at least one hollow cavity body, the sidewall of the cavity body is of a multilayered structure, a closed cavity containing pressure air is formed between every two layers of the sidewall, the cavity body is in a diameter-variable cylindrical shape with two thicker ends and a thinner middle part, and a vent hole is arranged on each cavity, the spring is hollow and therefore good in top-to-bottom air penetrability, the inflation amount of the cavity is adjusted in real-time by means of inflation or deflation via the vent hole in order to adjust the hardness (elasticity) of the air pressure spring and to enhance the comfortable level and applicable scope; after the pressure air in the cavities is eliminated and the diameter-variable cylindrical air pressure spring is compressed, the spring takes up a small space and can be coiled to facilitate package, storage and transportation; and cavity columns may be independent, or may be formed in rows or in lines, and the cavities of the cavity columns may be either independent of or communicated with each other.