The passive ankle exoskeleton can provide an efficient assistance at ankle push-off. However, current research mainly focused on its assistance effects during treadmill walking, the actual motion performance during overground walking has not been explored. In this study, we developed a low-cost passive ankle exoskeleton, and firstly investigated its effects on muscle activations and joint kinematics during overground walking. Five healthy young subjects were enrolled in our experiment where they were required to perform overground walking under three wearing conditions: normal walking, wearing an exoskeleton, and wearing an exoskeleton with zero torque assistance. The joint kinematics and electromyography ( EMG) signals of the major muscle groups of both lower limbs were collected simultaneously. Features of muscle activation and joint angles were extracted. A two-way analysis of variance ( Two-way ANOVA) was employed to investigate the effect of wearing condition on these features. The results showed that the mechanical frame can support the human body resulting in reduced muscle activation of medial gastrocnemius (MG) during the stance phase. Compared to normal walking, the exoskeleton significantly decreased the peak torque and power of the ankle joint while the average activation of the MG was decreased by 20% during the stance phase. Additionally, the exoskeleton may change the hip and knee joints' kinematics. The study demonstrated that the passive ankle exoskeleton effectively assisted the ankle joint during the stance but the wearer may alter gait performance for adapting exoskeleton.