Abstract: A method of X-ray spectra construction was founded based on the principle of least square algorithm. 14 different basic spectra were generated with accelerating voltage from 35 kV to 100 kV, and used in construction of the target spectra. According to the result, the similarity between constructed spectra and target spectra can be controlled to 90% or higher. The main influence factors are lack in quantity of basic spectra, the distribution of target spectra, and X-ray fluorescence produced by X-ray source.