An LD directly-pumped solid-state laser is considered to be one of the most promising mid-infrared light sources because of its simple principle， small size， and compact structure for the generation of mid-infrared （MIR） lasers in the 3~5 μm band. However， the quantum defect of LD directly-pumped MIR solid-state lasers will be much larger than that of ordinary near-infrared LD pumped solid-state lasers， which may lead to thermal damage and limit their development. In order to solve this problem， the methods of reducing the specific surface area of the crystal and improving the thermal energy released by the crystal structure are discussed， and the optimal length of the laser crystal is determined. The cooling structures of barium yttrium fluoride laser crystals （Ho³⁺：BY₂F₈） of different lengths were studied by thermal simulation using COMSOL software. The experimental results show that the output power can be increased and the thermal stress in the laser crystal can be alleviated by using the laser crystal whose length is slightly shorter than that of the cooler. The final experiment shows that when the pump repetition rate is 15 Hz and the pulse width is 90 μs， the single pulse energy is 7.28 mJ at the output wavelength of 3.9 μm， which is about 3 times as large as that of the crystal with the length of 10 mm （2.81 mJ）. Such results should be another breakthrough of our team since the first directly-pumped solid-state MIR laser was realized more than a year ago. It might pave the way for the construction of a feasible MIR laser in the near future.