Adopting two typical assemble nozzles which called taper nozzles and convergence nozzles as targets, the three-dimensional and plane symmetrical impinging jet model of laser cutting comprised workpieces is established and the flow field of assist gas is computed by adopting the integral form of N-S equation and RNG k-ε onflow model. The flow structure of assist gas and study the influence of standoff distance on flow field in cutting slot are calculated. The investigation reveals that the dynamics in cutting slot of convergence nozzle is more predominant than that in taper nozzle and at least two reasonable cutting scopes exist to make the assisting gas in cutting slot have predominant dynamic characteristic. In the end, laser cutting experiment and analysis are shown.