Deriving and analyzing the governing equation of the molten layer on cutting front with theory of the boundary layer and the perturbation method, we consider that a sta-ble molten stream need to be driven by friction forces but not the pressure gradient of the gas flow,or the pressure gradient is second factor. The flow feature of Laval nozzle is just as the above mentioned condition, taking account of the special demand in accordance with laser proccssing, we derive the design regulation and procedure of Laval nozzle used in laser cutting, and the area of operating parameter which guarantees the work normal of the nozzle.