Under the conditions of crankshaft uniform rotation, the schematic diagram of coordinate polishing crankpin was analyzed and found that the grinding time and speech changed, when the abrasive grinded crankpin. At the same time, the surface roughness was the relevant with the grinding time, grinding for a long time the roughness value was low, grinding time was short then the roughness value was high. In order to ensure that the surface roughness was homogeneity, so that the abrasive grinded the grinding point in the same time, in other words, the abrasive grinded crankpin in the constant velocity. And then the abrasive grinding crankpin was analyzed in the constant velocity, the crankshaft angle was draw the relationship between horizontal displacement and the swing angle. Simultaneity, adopted the level cylinder and the oscillating cylinder mutual moving, that resulted in the mechanism following the rotation of crankshaft back and forth movement, it was due to the weight of mechanism and the lag of the cylinder, and finally the mechanism of lag error was analyzed.