This paper proposes a new cable-driven wearable index finger exoskeleton to exercise flexion and extension of the index finger. This exoskeleton adopts an opportune constraint planar four-bar mechanism with an actively driving translational joint to reduce vertical space occupation and to achieve joint axis alignment for human-robot kinematic compatibility. The mechanical structure of the proposed exoskeleton is further optimized to minimize the exoskeleton's volume. The proposed exoskeleton is fabricated by 3D printing, has two active degrees of freedom (DoFs) configured in the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints individually, and can be easily worn by buckling the Velcro straps. Moreover, to ensure patients move their fingers freely after powering down, a decoupling mechanism is designed to remove the coupling between the motor and the exoskeleton. Due to the fact that the human's hand main tremble unconsciously or pick up a load suddenly, such unexpected disturbances may influence the control performance of the proposed exoskeleton. To this end, the active disturbance rejection control (ADRC) algorithm is adopted to reduce the influence of external disturbances. Finally, the functionality of the proposed exoskeleton is verified by experiments, and the experiments also validate the performance of the ADRC algorithm by comparing to the traditional proportional control.