Abstract: Radio-frequency ion thruster has been widely used in aerospace field because of its simple structure, precise and adjustable thrust and long working life.The ion density of discharge chamber is the key factor of the thruster performance.Based on the process of inductively coupled discharge, a two-dimensional axisymmetric fluid model is established.The variation of ion density in the discharge chamber under different aspect ratios and different structures are studied respectively.The results show that increasing the RF power and pressure can increase the ion density.The ion density is higher when the length of discharge chamber is 3.5-4 cm.Under the same diameter of the discharge chamber, Model 1 of cylindrical structure and Model 2 of frustum structure have the same volume.Reducing the surface area can improve the ion density.Model 3 of calotte structure, Model 4 of calotte-cylindrical structure and Model 1 of cylindrical structure have the same total length.Compared with the single cylindrical structure in Model 1, the optimal composite structure Model 4 has an ion density increase of 18.2%.When their densities are equal, Model 4 saves 15% of power input and 10% of pressure in the discharge chamber.