Abstract: In order to further study the effect of ultrasonic frequency on the stimulated cracking effect of coal, the paper conducted the crack experiment on the original coal samples by using the ultrasonic generation meter, and measured the coal by using the NMR equipment through analyzing the T₂ curve, the changes of pore structure before and after coal sample cracking were deeply studied. The results show that the pore structure of coal can be effectively improved by the ultrasonic wave. After the crack induced by the ultrasonic wave, the pore quantity inside the coal increased and the total porosity, effective porosity and permeability all increased significantly. Meanwhile, the growth rate of the number of small pores in coal is negatively correlated with the frequency of ultrasonic wave, and the growth rate of the number of medium and large pores, total porosity, effective porosity and permeability are positively correlated with the frequency of ultrasonic wave. The results of this study provide a good theoretical value for ultrasonic stimulation to crack coal seam and reduce the difficulty of coal seam gas extraction.