Abstract: Based on a COMET MXR225/22 X-ray tube, in two cases of big and small focuses, the original spectra were measured at different X-ray tube voltages by a HPGe detector. At the same time, the initial transmission spectra were measured under small focus. On this basis, the transmission spectra of X-ray through several different materials (Al, Ti, Fe and Cu) were measured. The results show that the spectra of big and small focuses can be approximately same, the low energy side has a large proportion, and the characteristic peaks of copper, silver, tungsten and lead are showed in original spectrum. The low energy side of the initial transmission spectrum hardens obviously, and the proportion of the high energy side increases with X-ray tube voltage. The characteristic peak of tungsten becomes evident, the X-ray average energy increases, and the maximum intensity area of the spectrum is near the 1/3 of the highest energy. The metal absorption sheet hardens X-ray evidently, and the hardening effect strongly depends on its atomic number. The results are applied to the hardening correction of industrial CT, and the quality of the reconstructed image is improved.