The invention relates to a gradually varied inertia liquid filling execution mechanism and a method for controlling a high-accuracy attitude of a spacecraft. The gradually varied inertia liquid filling execution mechanism comprises a vacuum container (1) for fixing and installing a whole flywheel structure, a flywheel bearing (2), a flywheel motor (3) and a flywheel. The control method comprises the steps that a laminar flow boundary layer equation is established, velocity distribution in a boundary layer is calculated, and an attitude kinetic equation of the spacecraft in the inertia changing process of the execution mechanism is established to obtain control toque Tc required by attitude change and provided by the execution mechanism. By the adoption of the gradually varied inertia liquid filling execution mechanism and the method for controlling the high-accuracy attitude of the spacecraft, the executing capacity of the attitude control execution mechanism is improved in the attitude control process, the output torque coverage range is widened, and meanwhile the problems of dynamic unbalance, too large resistance and the like brought by structure and process design problems during high-frequency rotation of a rotation part are decreased.