Semi-fixed abrasive plate (SFAP) lapping is an effective ultra-precision lapping method. It can efficiently obtain good surface quality in ductile regime machining. Probability density function of abrasive grains cutting depth is the key factor to control removing mode. This paper focused on the relationship between processing load and downward movement of the workpiece, which was important equation in probability density formula of abrasive grains cutting depth. By using loading test on pressure tester and numerical fitting method, the relationship can be expressed by different order polynomial formula, and the fitting curves were very close to experimental data.