Insoluble dietary fiber of potato slag was chosen to modify with superfine grinding using colloid mill. Inter-tooth space,of colloid mill, material-to-liquid ratio, treatment time are optimized by single-factor and orthogonal designed experiment and are defined with solubility process. Results shown optimum modified conditions of colloid mill: inter-tooth space 8.00 μm, 3 000 r/min for 5 min, material-to-liquid ratio 1∶60. Under these conditions, the solubility is 18.33%.