A systematic research on the gain-switched thulium-doped fiber laser is carried out in this paper. Based on the rate equation and the propagation equation, numerical models of a gain-switched thulium-doped fiber laser oscillator and an amplifier are constructed, and the solution is obtained by the finite difference time domain method. The characteristics of 2 μm laser output under different pump light and laser structure parameters are explored theoretically and experimentally. Through numerical simulation and experimental optimization, the nanosecond laser output at 2 μm band with high conversion efficiency, narrow linewidth, and single polarization is obtained. The seed source oscillator obtains the highest power of 796 mW, pulse width of 67.9 ns, and slope efficiency of 54.4%. A 2 μm pulsed laser with the highest power of 9.13 W and pulse width of 50.5 ns has been obtained after the first-stage amplifier. Simulation results of the numerical simulation model agree well with the experimental results. The model can provide a reference for the experimental research and engineering design of this type of laser.