While the alpha spectrometer workstation in the Radon Laboratory of Universityof South China is used to measure radon daughters under vacuity, about 36% - 37% of RaB, the decay daughter of RaA, has lost because of recoil. Without taking measures, the recoil will impact on the measuring accuracy of radon daughters and cause contamination of the measuring chamber. The experiment has effectively eliminated the RaB' s loss by using the Mylar membrane with a thickness of 1.5 impact on the meas...