Abstract: In order to achieve constant theory output flow of roots rotor pump at any speed, from the unique sinusoidal function square characteristic of the theoretical output flow of cycloidal rotor pump, a new structure scheme of double rotor pairs with four identical cycloid rotors assembled by axial dislocation and spacing on was derived, whose two theory output flows were integrated to desired constant theory output flow and its no theory flow ripple. All results showed that the cycloid profile was only option to achieve constant theory output flow or no theory flow ripple, the new double rotor pairs with 2 straight lobes and following 0.25 of roll diameter ratio and 45° of axial dislocation angle maximized the dimensionless constant output flow of 2.25; which was simple in structure, easy to be processed, no residual clearance, and high volume utilization, etc.