Based on the energy transfer between ions and the transition between energy leve ls of diode_end_pumped Tm,Ho:YLF laser, under the consideration of energy transf er up_conversion, the threshold rate equations are given, and the analytical for mulas of laser threshold are deduced from rate_equations. The relations between laser threshold and laser designing parameters are derived. The dependence of en ergy transfer up_conversion on the laser designing parameters is analyzed in det ail. Experiments on Tm,Ho:YLF crystals are performed to validate the present mod el.