In this paper, we have mainly studied the amplification effect of thulium-doped fiber amplifier (TDFA) at 2 µm, and compared different amplification effects of the one-stage TDFA, two-stage TDFA and three-stage TDFA at proper conditions. The simulation results show that within the effective threshold, with the increase of the pump power, the amplification effect of the optical amplifier improves, but the signal-to-noise ratio (SNR) of the output signal decreases, in order to balance the gain benefit and noise coefficient of TDFA, we can use a multi-stage amplification structure. Three-stage backward-pumped series 2.06 µm TDFA, whose slope efficiency can achieve 11% at certain condition. At 5.2 W pump power, the output signal gain of 2 µm TDFA exceeds 20 dB, and the output SNR is higher than 32 dB. In addition, the effect of the optimum length of thulium-doped fiber on the amplification performance of 2 µm TDFA is also analyzed in this paper. These simulation results are important for the experiment and design of 2 µm TDFA.