Tangential velocity indicator n of free vortex zone in cyclone separator has a significant impact on flow field and pressure drop inside the cyclone. Scholars have presented different n values on the basis of their experiences and hypothesis to guide the design of the cyclone separator. Until now, there have been a few researches on the n value, but no universally agreed conclusion has been reached. In this article, the specific n value is deduced by calculating the tangential velocity distribution inside the cyclone with numerical simulation method, followed by the curve fitting of tangential velocity of the quasi-free vortex zone with least squares algorithm. The influences on n value by diameter of the cyclone, diameter of exit pipe and its immersion depth are investigated. The results show that in the same cyclone, the change of radial position and the inlet flow rate have no effects on the n value; the changes of the axial position have very little influence on the n value; with the increasing of the diameter of cyclone, increasing of the diameter of exit pipe and the decreasing immersion depth of the exit pipe, the n value increases. (C) 2014 Elsevier B.V. All rights reserved.