In this paper, the fluid is filled with round hole in the workpiece, the piston is used to push the abrasive flow in the circular hole, and the piston is squeezed by one stroke for one cycle. The same workpiece with the same fluid abrasive cycle processing, the same point can test the diameter of the amount of change. And the change of the diameter of the measuring point is substituted into the modified kp parameter formula of the Preston equation based on the soft abrasive stream. The soft abrasive stream under different mass fraction is measured based on the kp parameter of the preston equation and Calculation. By analyzing the kp parameters of the soft abrasive flow under different mass fractions, it is concluded that the kp parameter of the Preston equation of the soft abrasive increases with the increase of the abrasive mass fraction, which shows the material removal ability on the microporous surface More and more strong. With the increase of the abrasive mass fraction, the removal rate of the soft abrasive grain is larger.