In order to lower the harvesting loss of alfalfa crop during the harvesting process, and optimize the working parameter of mower conditioner, the research of harvesting quality influenced by rolls and disks speed was conducted. A virtual prototype of machine and a mechanical model of plant were built by ADAMS software to simulate the process that alfalfa being cut, fed and conditioned. During the simulation at different rolls and disks speeds, average contact force between disks and plant, average contact force between rolls and plant, feeding time and conditioning time were recorded, and fitting equations reflected the relationship between them and working parameter of mower conditioner were built. According to the equations, a crush coefficient model was defined, which can reflect the degree to which the plants would be crushed by machine. As the crush coefficient model shown, when the disks speed ng was 1875r/min and the conditioner roll speed ny was 749r/min, the damage of plants caused by machine can be minimized. Field experiments were conducted by using different ratios between disks speed and rolls speed, and real damage rate was measured, the result verified the linear relationship between crush coefficient and real damage rate, in this case, the determination coefficient R2 was 0.95876.According to the field experiments and crush coefficient model, the lowest theoretical crush rate was 8.38%, which was 3.97 percentage points lower than the original crush rate, thus the fresh weight harvesting yield can be increased by about 0.47t/hm2, which was 4.53% higher than before