In view of the current situation that the research of alfalfa harvesting equipment in China is not enough and the quality of alfalfa is not high, combined with the agronomic characteristics and physiological characteristics of alfalfa, a new type of double rotary cutter was designed. The device can be hung in front of a tractor or self-propelled mower through the use of external stent and it also can work combining with the flattening rollers to finish mowing, flatting and gathering. The device integrated the nonstandard cutter, conical shaped rotating cylinder assembled guide device, circular cutter, sliding palm, transmission system, the four connecting rod lifting device and a fixed bracket. Round trip cutting can ensure smooth cutting and a certain height of alfalfa. The material would be promoted under the effect of the conical shaped rotating cylinder, and the height value and the angle between the horizontal plane and generatrix of conical shaped rotating cylinder were trigonometric function. The field performance experiment showed that the recutting rate, miss cutting rate and grass-crushing rate of two-disc rotary mower were 0.8%, 0.19% and 0.3%, respectively, which were 46%, 24% and 40% lower than the standard technical requirements. All the parameters of performance experiment were in higher level than the standard of GB/T 10938—2008 rotary mowers. And the cutting quality can satisfy the technical requirements.