Absorption spectra, emission spectra of Er3+ and Yb3+ codoped tellurite glasses were studed. Er3+ upconversion mechanism in tellurite glass was analyzed. The Judd-Ofelt strength parameters Ω t(t2,4,6), which are Ω 24.74×10 -20 cm2, Ω 41.46×10 -20 cm2, Ω 6 0.64×10 -20 cm2 respectively, were calculated by Judd-Ofeld theory. Spontaneous radiative rate and branch ratios of each energy levels of Er3+ ions were given. Stimulated emission cross section was calculated by McCumber theory, and was compared with other host glasses.