To investigate the origin of the thermoluminescence at high temperature, Lu2SiO5∶Ce0.006(LSO∶Ce)phosphors were prepared by sol-gel chemistry in air and graphite respectively. The patterns of X-ray diffraction, excitation and emission spectra and thermoluminescence curves of these samples were analyzed. In comparison with the phosphors prepared in the air, the luminescence intensity was improved and the thermoluminescence peak at 598 K was restrained for the sample prepared in graphite. It was found that the thermoluminescence peak at 598 K was relevant to Ce4+. In order to achieve more evidences, LSO∶Ce0.006,Kx(x=0.01～0.08) phosphors were prepared. The structures and spectra of K+-codoped samples were also studied. By codoping with K+ ions, the luminescence intensity of LSO∶Ce phosphor could be enhanced distinctly, but the crystallinity was not improved. The results of the K+-codoped LSO∶Ce exhibited that K+-codoping could improve the concentration of oxygen vacancy and restrain the peak at 598 K. When K+ ion occupied the position of Lu3+, more oxygen vacancies could be created and Ce4+ turned into Ce3+ simultaneously due to the balance of electric charge, which was consistent with the result of the thermoluminescence. So it can also come to the conclusion that the thermoluminescence peak at 598 K was relevant to Ce4+. The reason for K+ ions enhancing the intensity of LSO∶Ce can be attributed to the improvement of concentration ratio for Ce3+/Ce4+.