Abstract: The thread connections invalid of API tubing is the main form of tubing string failure according to the statistics from oilfields.Study on thread connection failure has been mainly focused on the force and deformation analysis under loads,few have been related to the flow field mutations of threaded connections.Therefore,it is necessary to study the failure mechanism by flow field mutations.In this paper,based on the computation fluid dynamics,governing equations of flowing fluid in thread connection have been built.Then,flow characteristics(distribution of pressure and velocity)in the connection of API round thread tube were simulated by FLUENT.Finally,technical measures that can improve the flow mutations have been proposed.According to the simulation and calculation of flow field in the "J"-shaped area,it was concluded that vortex and pressure mutation induced by high-speed flow would affect the fluid properties,leading the fluid to attack the inner wall of the connection.The newly designed API round thread could avoid the mutation of pressure and velocity,which was favorable for fluid flow,providing theoretical basis for the optimization design of tube.