Abstract: By means of the conception of projection rack, this paper presents a new simple and comprehensive method for investigation on the conjugation of helical gears in space-The Method of Medium Rack in Space. In this paper the fundamental principle of the Method of Medium Rack in Space is introduced. And by means of this method, the general basic formulae of conjugation in space (such as the formula of the conjugate curved surface,the formula of the path of contact etc) are produced in a simple form.In this paper, the conjugation of helical gears is classified into two cases-the normal conjugation and the non-normal conjugation. By means of the method, developed, an important rule has been discovered, that is, "In the case of normal conjugation, the path of contact of helical gears is a plane curve", This path of contact lies in the normal section of the gears and passes throagh the "pitch point". It means that the conjugation of helical gears in space takes place in the normal section, and the character of the conjugation in space is very similar to the conjugation in plane, In this paper, the character of non-normal conjagation of helical gears is also discussed.The helical gears discussed in this paper are of any tooth form.The method developed in this paper provides the theoretical basis for cutting helical gears.