Design theory of high order intermittent mechanisms is an important branch of mechanism design.Mathematical analysis manifests that that one order differential of two correlative functions in a compound function at same moment is zero leads to one to three order differentials of the compound function at corresponding moment to be zero.This a principle sets up a new method on high order intermittent mechanisms.Research indicates that this a mathematical principle corresponds to tens kinds of planar or spatial mechanisms which driven members at limit position（s） do unto three orders intermission.For this reason

it is only introduced that a type of seven-bar mechanisms of the slider at limit position（s） does unto three orders intermission based on crank-rack and its design

its transmission characteristics are studied.The mechanism has the simpler structure and its design is easier and transmission angle is larger.