The existing passive exoskeletons have some defects, including the lack of assistance effect. Based on the gait characteristics of human body, the principle of transforming gravitational potential energy during walking process into kinetic energy during lifting leg process was proposed, in which a passive lower extremity mechanical exoskeleton was designed by the combination of spring, crank slider mechanism, ratchet pawl mechanism and chain drive mechanism, and the performance of exoskeleton was evaluated by kinetic analysis and physiological cost index experiment. Results of kinetic analysis show that the total peak torque of hip joint is reduced by 23.38% and the total work of hip joint is reduced by 30.59% after wearing the exoskeleton, which preliminarily verifies the effectiveness of the exoskeleton. The results of experiment show no significant effect in short distance action conversion, but is reduced by 8.1% and 10.4% in long distance walking and uphill walking, respectively. Therefore, the assisting performance of the passive exoskeleton has been verified, which provides an instance reference for research in related fields.