The theoretical studies of the upper state population inversion ratio for erbiumdoped tellurite-based fiber amplifier（EDTFA） are presented, and its distribution curves with the activated fiber length, each channel signal input power, bi-directional pumping power and erbium-doped concentration, respectively, are also obtained. Investigations of the relations between the population inversion ratio and the signal gain of EDTFA are given. It is shown that the signal gain of EDTFA is determined by the upper state population inversion ratio.