The preliminary experimental results obtained for the energy spectra of high-power ion beam(HPIB) with a pile of thin films on FLASH Ⅱ accelerator are reported. Code was developed to calculate the theoretical energy-loss of proton and C ion beams transmited through the Mylar films with different thickness. T he apertures of Faraday Cups are covered with Mylar films of different thicknes s, and then the signal attenuations can be recorded with oscilloscope. The energ y spectra of HPIB and the diode voltage are obtained according to the signal al lenuations. The diode voltage measured with a pile of thin films agrees with the voltage measured with a differential ring. And the ion number versus time curv e was given. In addition, the ion energy of HPIB is also measured with Thomson spectrometer and time of flight method and the results agree with those measured with a pile of thin films.