Tm (6%), Ho (0.5%):YLF laser cooled by liquid nitrogen pumped by a laser diode with 792 nm wavelength was studied in theory and experiment The maximum 350 mW output power was obtained, and optical-optical conversion efficiency is 20% corresponding to 28% slope efficiency. The threshold power and output power dependence on the temperature of Tm,Ho:YLF crystal and the wavelength of LD were also studied.