Through measuring the characteristic X-ray for copper target of mass thickness 23m g/cm2, its K-shell ionization cross section was calculated. The method of thin target with thick substrate was used in the experiment. The influence of the electrons reflected from the substrate was corrected by means of a electron transport calculation. The multiple scattering effect for the copper target of mass thickness 23m g /cm2 is corrected by the EGS4 program of Monte Carlo techniques. This method is reported for first time.