Cellulase is potential tool for modification of pulp properties to save energy requirement during refining process. In this paper, bleached Simao Pine kraft pulp was treated with NOV476 cellulase under different enzyme dosage for action mechanism analysis. Pulp viscosity was measured to evaluate the damage of fiber by enzyme. Fourier transform infrared spectroscopy (FT-IR) was performed to study the enzyme treatment on the pulp chemical structure and crystallinity index. The fiber morphology difference before and after treatment was also revealed by the SEM and AFM observation. When the cellulase dosage was lower than 1u/g, the fiber mean length slightly increased. With the further increase of cellulase dosage beyond 1u/g, the fiber mean length was less reduced than the control sample. The fiber mean width increased steadily after enzyme treatment in the dosage range of 0~0.1 u/g. Enzyme treatment did change the fiber chemical structure and crystallinity index through FT-IR analysis. With the increase of NOV476 cellulase dosage, the pulp viscosity decreased steadily. The SEM analysis showed the surface of cellulase treated fiber had some fluffing phenomenon and there was obvious intertexture between fiber and fiber. AFM images further demonstrated that the fiber surface of controlled pulp was covered by primary layer. After enzyme treatment, the primary layer of fiber cell wall was peeled off.