Abstract: According to the principle of the laser spot size requirement and the design principle of Laval nozzle,the part of stable section,contraction section,throat section and expansion section of the nozzle were designed.The size of stable section is mainly limited by the overall size of the laser cutting gun.Contraction section is used to connect the stable period and the throat section by using tangent arc transition.In order to ensure the laser go smoothly through the throat,throat diameter must be greater than the laser spot diameter.Expansion section adopts linear expansion method,in view of the cutting seam width limit,divergence angle should not be too large.According to the design size,the supersonic Laval nozzle was manufactured,the jet velocity and stiffness of oxygen flow was improved.In underwater laser cutting experiment,using the local drainage method and larger oxygen flow,30 mm thick carbon steel plate was cut smoothly.Compared with the convergent nozzle, the cutting effect of the supersonic nozzle has been obviously improved.