This paper presents the design and preliminary evaluation of a portable spring-based knee exoskeleton, the SpringExo, which is designed to provide assistance to the leg while minimizing interference with the natural leg movement. Traditional rigid exoskeletons are unable to accurately align with a user’s anatomical joints. In addition, the user’s natural motion pattern is likely to change due to the constraints of the rigid exoskeleton. Though some textile-based soft exosuits and cable-driven soft exoskeletons have been developed to achieve better alignment with human’s biological joints, forces applied by the cables have to be sustained by human skeleton and joints. SpringExo, in comparison, uses a coil spring which the user wears around the thigh and shank, and does not require alignment with the joints. The spring stores energy and provides minimal interference during elastic deformation. A key feature of the SpringExo is that the springs store energy during the flexion phase and release this energy to assist the knee extension in the extension phase. We conducted human subjects study to verify its biomechanical and physiological effects on the user during stair ascent. Results from a six-subject study showed that the device did not interfere with the natural joint angles and assisted knee extension during stair ascent. However, further redesign and optimization are needed on the actuation system to offset SpringExo’s drawback of hindering knee flexion.