Theorem of choosing teeth is one of basic rules of designing screw steel ball moving teeth driver. It is the frist time to illustrate the rule that the line number of helix of the input axis and output axis could determine rationality of a design in this paper.The greatest common divisor of these axes'helix line number should be designed as great as possible for the purpose of obtaining the greatest carring capacity.Examples and comparison of application of these laws was given finally.