A temperature model for a end-pumped thin disk laser with Gaussian beam intensity distribution and single pass absorption performance is built up according to the actual working state. Using infrared thermal image, temperature distribution, temperature variation with time and the temperature difference of the surface laser medium are measured at different pump powers. Experiment on thermal lens focal length of the thin disk laser medium is performed by utilizing Hartmann-Shack wave-front sensor. Considering the fact that thermal lens focal length of the laser medium changes with pump power, the model for analyzing the influence of thermal effect on output power of thin disk laser is presented, according to rate equations. The simulation results obtained in this paper show satisfactory agreement with experimental results. The research has the guiding significance for designing and optimizing thin disk lasers