Abstract: The attenuated law of flow field inside the nozzle block of the air jet vortex spinning is investigated by the method of numerical computation.Force analysis about a separated fiber in twisting chamber and the movement of a separated fiber affected by vortex in twisting chamber are discussed.The function relationship between the critical angular velocity of the fiber′s open-end twined over the hollow spindle and rotated around the air jet vortex spun yarn and the length of the leading fiber ends inside the yarn′s trail,fiber radius and so on is constructed.The critical angular velocity may judge whether the leading fiber ends pulled out the yarn′s trail or not,which can explain the generation of waste fibers and thin places.The critical angular velocity increases along with increasing the length of the leading fiber ends inside the yarn′s trail.On the condition of the same nozzle pressure,the fiber radius is bigger,waste fibers and thin places of the air jet vortex spun yarn are severer.Increasing nozzle pressure firstly can enhance the yarn strength,however,when the nozzle pressure exceeds the critical point,and waste fibers and thin places become more,which make the yarn properties deteriorate.Increasing negative pressure inside the nozzle and decreasing properly the distance from the front roller to the hollow spindle can decrease the generation of waste fibers and thin places.