Based on the fiber waveguiding theory and hormgeneously broadened four energy-level model,the designing considerations of the fiber structure parameters-fiber core radius and relative index difference of tellurite-based erbium-doped fiber with step-type index distribution,which is applied in broadband amplifiers,are presented theoretically. It is shown that the tellurite-based erbium-doped fiber with bigger relative index difference is needed to form fiber amplifier in order to get larger signal gain under single-mode conditions. While,for the same relative index difference,the bigger the fiber core radius,the larger the signal gain when in the state of high pump power,low signal input or shorter than the best fiber length. On the contrary,the smaller fiber core radius is needed. The above results will provide theoretical significance for the fabrication of new type broad-band tellurite-based erbium-doped fiber.