There are many materials about the method of determining the diameter D1 of the front Journal of the main shaft, but generally speaking, either the range given is too large or too complicated. Especially in the initial stage of design, because the bearing and its position are not determined, there is not a precise and effective simple calculation method. According to the working practice for many years, it is considered that when starting to design the main shaft, the diameter D1 of the front Journal of the main shaft can be calculated initially with the empirical formula of torsional rigidity, and then the relevant parameters can be determined successfully through analogy analysis. This method is simple and applicable. It is introduced as follows.