A single frequency admittance spectroscopy technique is introduced for determining the band discontinuity on Si/GexSi1-x/Si single quantum well (SQW). By using this method, the more accurate experimental results can be obtaind from a single frequency admittance spectrum. For an alloy with composition x=0.33，we get the activation energy Ea＝0.20eV. In order to obtain the value of the band offset，it is necessary to determine the position of Fermi level which depends on the dopam concentration inside and outside the well, the band offset of hetero-interface and temperature. By solving Poisson equation, we get the energy band diagram and Fermi level of the test sample. It is shown that the Fermi level lies about 40 meV above the top of valance band in the well. And due iv the carrier transfer at the hetero-interface, the carrier concentration (about ２×１０17ｃｍ－３) in the well is one order large than dopant concentration(1×１０16ｃｍ－３). The energy bands at two sides of the interfaces are bent by about 0.006 eV in the well and O.11eV in the barrier, hence the valance band offset can be calculated as 0.16eV. The accuracy of this technique can also be verified from reconstruction of C-T and G-T curves by cumputer simulation.