Output power characteristics of thulium-doped fiber ring lasers are studied numerically through Matlab programming with a theoretical model developed based on the fundamental atomic rate equations and power propagation equations of the simplified two-level laser system and the homogeneous broadening theoretical model. The effects of total intra-cavity loss, active fiber length, output coupling ratio, pump wavelength and pump power on output power and laser emission wavelength tuning range are studied. The numerical simulation results show that minimization of intra-cavity loss and increase of the pump power as well as optimization of active fiber length is very important for the thulium-doped fiber laser to improve the output power and increase the wavelength tuning range. In the meantime, increase of the output coupling ratio can also improve the laser output power, but reduce the wavelength tuning range. A broad wavelength tuning range up to 528 nm (1660~2188 nm) is achievable by increasing the pump power and optimizing the length of thulium-doped fiber when the total intra-cavity loss and output coupling ratio are 3 dB and 10%, respectively. It is much higher than the experimental and simulated results reported so far. Partial 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.