Based the energy transfer between energy levels and the transition between ions of diode-end-pumped Tm,Ho:YLF laser, with consideration of energy transfer up-conversion, the rate equations of Tm,Ho:YLF Q-switched laser are given, and the analytical formulas of single pulse energy is deduced from the rate-equations. It is shown theoretically that the energy transfer up-conversion reduces not only the single pulse energy but also the upper level lifetime of Tm,Ho:YLF Q-switched laser. An experiment is carefully designed to verify these conclusions.