Abstract: The rotors’ profiles of screw pumps were complicated generally,and their basic profiles were several kinds of cycloids. There were several methods about the deduction of equations of rotors’ profiles, e.g. as the way of geometry, the analytics,etc. Vector algebra was adopted to deduce the equations of the rotors’ cycloid profiles of<135>three screw pump. The equations included the whole deducing course about the parameter equations of theoretical and practical rotors’ profiles and the ranges of parameter values. Intermediate course and relative concept was explained too. It was found that this method was simple and the steps were dear. It was suitable for engineers and technicians to understand and apply it.