Abstract: The α spectrometric measurement for radon progeny under vacuum results in the recoil loss of RaB. The recoil loss impacts on the measuring accuracy of radon progeny and causes contamination of the measuring chamber. Millipore AA type membranes (about 0.8 μm pore size) were selected and three different sampling flow rates were 6.3, 12.3, and 15.6 L/min respectively. ELPI particle size analyser, α measurer and α spectrometer were used for measuring aerosol size distribution of radon chamber, self-absorption coefficient of filter membrane and recoil loss rate respectively. The results show that the aerosol spectrum in radon chamber presents a single peak distribution; under the corresponding flow rates as mentioned above, the self-absorption coefficients are 0.978, 0.980 and 0.989 respectively, and the recoil loss rates are 0.362 8, 0.368 4 and 0.362 9 respectively. It shows no significant difference, but the RaB produced by the decay of RaA on the filter membrance samples has large recoil loss.