The work aims to design an ergonomic cushion and apply it creatively based on the passive dynamic sitting concept to alleviate the muscle fatigue and perceived discomfort of office workers induced by sedentary sitting. By studying the seated sports anatomy and the knowledge of passive dynamic sitting, an intelligent dynamic ergonomic air cushion with a specific movement pattern was designed. Through sitting experiments, the feasibility of the cushion and the movement pattern were tested through a surface electromyography experiment and a subjective discomfort questionnaire. The results demonstrated the mean frequency (MF) change values in the experimental group were larger than that in the control group, and the effect size was large in the right iliocostalis, bilateral multifidus, and bilateral longissimus (Cohen's f > 0.40), except for the left iliocostalis. The results also demonstrated the perceived discomfort (PD) changes in the experimental group were smaller than that in the control group, and the effect size was large in the lower back, buttocks, ischial tuberosity, seat edge, and overall (Cohen's f > 0.40). An intelligent passive dynamic health seat was proposed on this basis. The use of an air cushion and its passive dynamic intervention can effectively alleviate muscle fatigue and perceived discomfort caused by sedentary sitting, which is positive for improving sitters' health and comfort during sedentary sitting.