Field tests were carried out using New Holland HW320 Self-Propelled windrower on alfalfa soil during clover harvesting in order to check the compacting effects and the alfalfa yield changes resulted from one to ten passes of the mowing machine along the same track. Then the changes of the soil bulk density, soil hardness, under different passes and different depths were measured and porosity, three phases of soil were calculated. And the changes of alfalfa yield after different passes were measured. The results indicate that soil compaction makes alfalfa soil condition change remarkably, all parameters were influenced by mowing machine. The more the number of tyre passes, the higher compaction degree; mowing machine caused the changes of soil structure from 0 to 30 cm depth mainly; water permeability is the most one of sensitive factors, only six passes can cause saturated hydraulic conductivity of water reaching zero. As a result of that soil compaction caused by mowing machine can influence the alfalfa yield and the decrease of alfalfa yield is 4464 kg/hm² annually after ten passes as compared to the alfalfa yield of growth in the original field soil.