Abstract: When the pressure in the vacuum container of the intermittent type hypersonic wind tunnel rises up to a certain extent, the flow field in the test section begins to destroied, and the static pressure in the test section would be unstable. The pressure in the vacuum container at the corresponding unstable point is called the critical pressure. The critical pressure would affect the vacuum system design of large-scale hypersonic wind tunnels (Ma ≥ 10). The experiment is carried out in the Φ0.3m hypersonic low density wind tunnel, where the Mach number is over 10. The pressures in the test section, the pitot tube, the diffuser and the vacuum container have been measured. The critical pressure ratio of Ma10, Ma12 and Ma16 is 0.34, 0.35 and 0.5 respectively. The wind tunnel flow field unsteady process from establishment to destruction is simulated by FASTRAN software. The calculation result is in agreement with experimental result. The investigation provides key data for the vacuum system design of similar larger hypersonic wind tunnels.