Abstract: In the view of the distribution of X-ray spectrum, the explanation of quantum and the characteristic of material attenuation for X-ray, the arithmetic which simulates the projection of the consecutive spectrum was proposed based on the attenuation of X-ray intensity. First in this arithmetic the consecutive spectrum of X-ray was divided into several discrete and narrow spectrums. At the same time in order to realistically reflect the consecutive spectrum of X-ray, the method of subsection curve fitting was used to fit the function of spectrum on the every narrow spectrum and the function of attenuation coefficient with the X-ray energy changing. Then the projection was simulated according to the attenuation rule of the polychromatic ray. At last, the projection of X-ray which had the consecutive spectrums characteristic was derived by fusing projection. CT reconstruction by using the projection derived from this arithmetic showed the anticipated beam hardening effects. At the same time, this arithmetic realistically simulates the projection process of practical system of CT.