Abstract: In order to improve the extraction efficiency and extraction rate of coalbed methane to the maximum extent, the ultrasonic explosion of coalbed methane reservoir is used to improve the permeability of the reservoir. The evolution of pore size and quantity of coal in the process of ultrasonic fracturing coal was detected by nuclear magnetic resonance （ＮＭＲ） technique. The fissure development process of coal was monitored and analyzed by thermal imaging, digital camera and P wave rock measurement system. The results show that the porosity and the number of pores increase and the porosity increases by 120.5% with the formation of ultrasonic fractures. The whole fracturing process is divided into three stages, and different pore sizes sorted by time are “micropores, mesopores, macropores and fissures”; P-wave test shows that ultrasonic waves can stimulate the internal and surface of coal to form a fissure network, greatly improve the permeability of coal; because any water-bearing pores are caused by ultrasonic cracking fracture points, compared to the traditional fissure measures, ultrasonic fracturing has a wider influence range and needs less energy.