TeO₂-20ZnO-xLa₂O₃-0.8Tm₂O₃(x =0,5%,10%,15% in mole ratio) tellurite glasses were prepared by high temperature melting method.The effects of La₂O_3content on thermostability of tellurite glasses are analyzed by DSC curves.J-O strength parameters,spontaneous emission probabilities,fluorescence branching ratios, fluorescence radiative lifetime and other spectrum parameters of Tm³⁺ ion s in tellurite glasses are calculated with absorption spectra and Judd-Ofelt theory.The fluo rescence spectra are measured under the pumping wavelengths of 473nm and 808nm,re spectively. The effective linewidth of fluorescence and emission cross sections are calcul ated. The results indicate that the increase of La₂O_3content is beneficial to imp roving the thermostability of tellurite glasses (as ΔT=Tx－Tg chan ges from 123℃ to 180℃) and enhancing the fluorescence spectral properties of Tm³⁺ at 1.46μm.When La ₂O_3mole ratio is 15%,the values of the full width at half maximum (FWHM) and FWHM×σ_e can reach up to 113.2nm and 409.33×10－28 cm³,respectively,and it is a potential host material used for S band optical amplifier.