Based on the rate equations and light propagation equations of four energy level system,the performances of gain and noise figure of broadband erbium-doped tellurite fiber amplifier(EDTFA)with an isolator in the midway are researched. The results show that the signal gain and noise figure are obviously improved due to the effective suppression of backward propagating amplified spontaneous emission(ASE)noise and the resulting consumption reduction of reversed particles by the isolator. Meanwhile the analyzing of gain and NF regard to different fiber length and different isolator position shows that the gain improvement of about 10 dB and NF reduction of 1 dB are expected at the short wavelength when the isolator lies at the optimum position. Amplified bandwidth and PCE of EDTFA also be improved with a midway isolator.