Spacecraft, Attitude Control, Flexible Structure, Unsymmetric Structure, Torque Arm Abstract: Unsymmetric flexible spacecrafts have particular dynamic characteristics; when the attitude control actuator such as RCS thruster is used, the imbalance between negative torque and positive torque is caused. And even worse, an inertia matrix and a coupling matrix are changed depending on the displacement of the rotational paddle and the mobile sensor. In this paper, an attitude control system with consideration of these characteristics is proposed. Location of thrusters is designed to satisfy that the nominal control torque is larger than the imbalance torque. And control method includes PD, filter and variable gain, which maintain high quality of closed loop system. The validity of the proposed method is verified through the numerical simulation results.