Firstly, the characteristics of person-parachute sys-tem (PPS) in free-fall phase are analyzed. Then, the particle motion equations of PPS are built up. The motion law of PPS in free-fall phase is found by computing. Fall synthesis velocity decreases firstly, then gradually increases, so there is a minimum value of synthesis velocity when is favorable parachute opening moment. Results and actual test show good agreement, which provides an evidence for determining parachute opening moment.