The invention belongs to the aviation lifesaving field, and relates to a method of determining ejection speed of a man-seat system to determine a safe parachute opening speed condition when a pilot starts emergency ejection. The main technical characteristics are that according to a corresponding relation between an initial speed parameter when a man-seat system discovered and explained for the first time is ejected out of a cabin and a speed variation parameter after ejection as well as a corresponding relation between an acceleration parameter after ejection of the man-seat system and the speed variation parameter, a method of calculating and obtaining speed when the man-seat system is ejected out of the cabin on the basis of the mass, an aerodynamic resistance coefficient and an aerodynamic characteristic area of the man-seat system as well as an acceleration, an attitude angle, an altitude height, seat secondary power or the like after ejection of the man-seat system is designed. An ejection speed parameter can be obtained automatically and reliably, and a program control mechanism determines the time when a survival parachute is opened. Therefore, an ejection lifesaving success rate is increased, and lifesaving safety of the pilot is guaranteed.