The mode transition of combined-cycle engine makes it necessary to throttle the mass flow rate of propellants continuously and stably. Aiming at gaseous propellant, a throttleable sonic nozzle was designed on the basis of normal sonic nozzle. The throttleable sonic nozzle achieves continuous throttling through a plug moving along axis. By adopting the twice-envelope method, the contour of the plug was designed to satisfy the linear characteristic of mass flow rate. Performances of the throttleable sonic nozzle were investigated through the numerical simulation of computational fluid dynamics. It is found that the mass flow rate is independent under the back pressure when the back pressure is lower than the critical back pressure and the critical back pressure ratio increases as mass flow rate decreases. The linear relationship between mass flow rate of linear throttleable sonic nozzle and location of plug was verified and results show that the discharge coefficient is high and hardly be affected by the location of plug.