In precise testing and control occasions, a stable air pressure supply is often required. However, the use of pressure regulating valves has low accuracy and slow response, which cannot meet the requirements of high-precision air supply and is difficult to deal with leakage situations. A high-precision constant pressure control system based on Fuzzy PI was designed for small volume chambers by using a proportional valve with high frequency response. In order to verify the effectiveness of the system under leakage conditions, an additional proportional valve was connected to the chamber to carry out a series of experimental studies under three conditions of confinement, constant leakage, and variable leakage.The results showed that the designed constant pressure control system can adapt to a variety of harsh operating conditions. The maximum steady-state errors of the pressure control for the 10 L, 20 L and 30 L chambers were around 610 Pa, 550 Pa and 490 Pa under the condition of greatly varying leakage, respectively. The pressure control precision achieved was close to the precision of the pressure sensor itself, much higher than the precision regulated by the traditional precision pressure reducing valve, and much faster in response. In addition, the test results also showed that the designed controller has good adaptability to the chambers with a volume range of 10 L to 30 L.