A series of experiments on a black non-charring polymer in the low-pressure chamber is conducted under different external heat fluxes. The surface and bottom temperatures and the mass loss of the sample are measured. A parameter Tp is introduced to describe the impact of pressure on the surface temperature. There is a loose layer of the char residue left with significant pyrolysis bubbles under the low heat flux, and the bubble size decreases with pressure. The parameter Tp is found to exhibit a significant decline trend with increasing pressure, and the mass loss rate of the sample decreases apparently as the pressure increases. However, under a high heat flux, the char residue is denser, and the pyrolysis bubbles are not observed. The value of Tp and the mass loss rate of the sample have no obvious relationship with pressure. The average pyrolysis rate is linearly proportional to pa.