Abstract: In preceding design of cam, people didn't pay great attention to the dervative of ascending distance acceleration of cam (I. e. the third derivative of ascending distance curve), and it is also difficult to put their hands to it. In this paper, we consider this problem. Through using many optimum methods, calculation and comparison of many practical cams, we prove that the effect is very remarkable for solving the contradiction of "mechanical property" and "plentiful coefficient" of cam mechanism.