Ionic wind generators have shown great potential in applications in devices cooling, air actuating and flow control. Among various types of ionic wind generators, the needle-ring type is of particular interest due to high permeability of collector electrodes, flexible modification of structure and practical combination of multiple generators in a row or series. In this study, a three dimensional modeling has been conducted to study the electric and flow characteristics of needle-ring ionic wind generators. We numerically measured the discharge current and outlet air velocity with respect to applied voltages, gaps between electrodes, ring electrode's geometries and needle electrode's eccentricity. The results showed that voltage-current characteristics and outlet velocity were significantly influenced by distances between electrodes (G) and diameters of ring electrode (D). Hence, the effect of non-dimensional parameter G/D was investigated and the optimum ratio is 0.2-0.3, which agreed well with results from other literatures. The influence of eccentricity on efficiency was also studied and the mechanism was discussed in detail.