Abstract: This paper investigates the relationship between the position parameters P and Q of the legs of a quad-ruped walking robot.The △K boundary equal to P minus Q has been given.When the length is not cqual tothe width of motion space of a leg,a reasonable dafa of △K has been given.The stability margin S under theinfluence of the weight of legs and the speed of robot has been rcsearched,and the increment of stabilitymargin has been introduced.