Abstract: Thin silicon foils with thickness about 3 to 4 μm are prepared by semiconductor process combined with heavy doped self stop etching process. Boron impurity in thin silicon foils influences on the property of thin silicon foils. Secondary ion mass spectrometry is adopted to measured the distribution of boron impurity. Transmissivity of thin silicon foils is measured on Beijing synchrotron radiation facility by using the synchrotron X ray beam with the wavelengh from 12 0 to 21 0 nm.