In order to study automatic ultrasonic testing technique based on watersquirting for the curved surface workpieces, the special nozzle was designed and the numerical simulation of the flow field in the nozzle was investigated by using CFD (i.e., Computational Fluid Dynamics). The function of threelayer porous sieve structure in the nozzle was analyzed, and the relationship between the size of the annular hole of the sieve and the flow velocity of flow field was studied. The results show that the annular hole has an important influence on the flow state of the fluid in the nozzle. Choosing the suitable size of annular hole can obtain laminar flow in front of the ultrasonic probe which could provide a stable column between the probe and the workpiece.