In order to improve the laser welding quality, laser welding experiments were carried out, in which the 1CR13 specimens were treated by varied power of ultrasonic vibration. By the test of microstructures of welding joints grains, the effects of ultrasonic vibration on laser welding process were studied, some results were found that the growth of the columnar crystal grain was affected, with the growth of the ultrasonic vibration power the grain was refined. According to the metal solidification theory and ultrasonic vibration cavatition theory, we can conclude that the ultrasonic vibration cavatition causes the fall of temperature of micro zone and partial high pressure in the welding molten bath, which increasing the driving force of melt crystallization and reducing solidification activation energy, increasing the melt nucleation rate.