Abstract: In laser beam welding,an appropriate crossjet nozzle should be mounted under the welding head in order to protect the optics from the welding spatter and smoke.An ultrasonic crossjet with a non-symmetric Laval cavity is designed based on the aerodynamic theory.Schlieren trial show that a rigid flow with a small divergence is obtained,and no disturbing near the flow can be observed.Welding experiments prove that focusing mirror is well protected when such a crossjet is used.