Based the energy transfer between ions and the transition between energy levels of diode-end-pumped Tm,Ho:YLF laser, and considering the energy transfer up-conversion and ground state re-absorption, the rate equations are set up. The analytical formula of fractional thermal loading is deduced from the rate-equations, and the results show that the fractional thermal loading critically depends on the pump-to-mode size ratio. The focal length of the thermal lens as a function of pump power is obtained in experiment. The experimental results are compared with theoretical results, which shows that the theoretical results are reasonable.