Based on the data of AWS, radiosonde observations, satellite and Doppler Radar D ata, an analysis on the jets in a torrential rain event in Liaoning Province on July 31, 2008 was conducted. The results show that, a large scale southerly jet and a mesoscale southwesterly jet preexisted in the boundary layer backgr ound and in the left side of latter a southwestnortheast mesoβ cloud cluste r band caused the heavy precipitation directly. The further analysis indicates tha t the mesoscale boundary layer southwesterly jet was near to the high level je t in direction and speed. The bishear pattern wind field trigged the unstable d e velopment of vortex Rossby gravity wave in the left side of the boundary jet. It gave birth to the mesoβ cloud cluster band.