The theoretical model of thulium-doped fiber laser based on the simplified two-energy laser system and uniform spreading theory model was established, using the atomic rate equation and power transfer equation. Taking the ring cavity thulium-doped fiber laser as an example, the relationship between its output power and wavelength tuning range and cavity loss, thulium-doped fiber length, output coupling ratio, pumping wavelength and pumping power, etc., was investigated through Matlab programming numerical simulation. The numerical simulation results show that reducing the laser cavity loss, increasing the pump laser power and optimizing the thulium-doped fiber length can improve the output power and increase the wavelength tuning range of thulium-doped fiber laser, while increasing the output coupling ratio can improve the laser power but reduce the wavelength tuning range. After parameter optimization, the wavelength tuning range of the thulium-doped fiber laser can reach 528 nm (1 660~2 188 nm) by increasing the pump laser power and optimizing the thulium-doped fiber length at a total intra-cavity loss of 3 dB and an output coupling ratio of 10%, which is higher than the experimental results reported so far. Some of the simulation results are compared with those experimental results in the literature, and the accuracy of the model is well confirmed. These findings have important theoretical reference value and guiding significance for the design and development of thulium-doped fiber lasers.