A numerical model of the upconversion fiber laser with gain media of erbium-doped tellurite-based glass fiber was presented. The lasing output performance of erbium-doped tellurite-based glass upconversion fiber laser was investigated theoretically, and the dependence of 546 nm green output power on launched 980 nm pump power and fiber length of resonant cavity were analyzed. The results indicate that erbium-doped tellurite-based glass upconversion fiber laser can achieve milliwatt lasing power in a resonant cavity length of centimeter order, and its slope efficiency reaches about 7%. Compared to the reported fluoride glass upconversion fiber laser, the erbium-doped tellurite-based glass upconversion fiber laser shows a potential small-scale characteristic of solid laser.