According to the selfgeometric characteristics of the cycloidal gear, stringtop method, one kind of fast measurement method which fits to the cycloidal gears that own the characters of small modulus and more teeth, is put forword. The calculating formulas of the distance between the sting and the top of the cycloidal gears’ theoretical tooth surface are given，furthermore, the distance of the modified profile of the cycloidal gears under the conditions that moving distance, equal distance and rotating angle are also discussed. The calculating program is written, stingtop method of the cycloidal gear’s fast measurement software is developed, calculation and measurement example is given.