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This study introduces a wearable soft knee exoskeleton that aids active knee motions during walking. It is mainly driven by vacuum-actuated rotary actuators. In this paper, the overall design of the exoskeleton is introduced. Moreover, the design of the vacuum-actuated rotary actuator is introduced, and the correspondence among the interior air pressure, rotation angle and output force of the actuator under a vacuum condition and during the transition from the vacuum to an equal atmospheric pressure condition were studied. Then, the corresponding relations among the pressure, angle and torque of the actuator were obtained to construct a knee torque model. Furthermore, we introduced in detail the control system of the exoskeleton, including a gait estimation model and knee torque model. The control system analysed knee angle information measured by IMUs and the air pressure of the actuators measured by air pressure sensors. Then, by the calculations performed by both the gait estimation model and knee torque model, the corresponding commands for the aerodynamic switch, pressures and air flow rates were determined. According to the commands, a micro air pump and valves provided quantitatively positive or negative pressure for the actuators to generate gait-consistent auxiliary torques, stretching torques and bending torques that were able to meet the needs of the knee during walking. Finally, a cardiopulmonary exercise test was used to quantitatively evaluate the exoskeleton. The results show that under the same load condition, the metabolic cost of walking is reduced by an average of 6.85% when the exoskeleton is worn.