At present, paper industry has a lot of problems in China, such as the shortage of forest resources and high cost of pulping. Paper industry must be seeking high yield and low cost pulping process. In order to reduce the cost of mechanical pulp and increase the use of mechanical pulp, a lot of research and patent showed that the refining energy consumption of mechanical pulp would be further decreased using the corresponding enzyme to modify fiber of the raw materials before refining. The effects of cellulases modification on masson pine mechanical pulp were investigated in this paper. The modification conditions were as follows: the dosage of enzyme 75IU/g (to bone dry materials), temperature 500C, time 150min, pH value 5.5 and pulp concentration 15%. The effects of cellulase modification on the surface and quality of masson pine fiber of mechanical pulp were investigated by scanning electron microscope (SEM), atomic force microscopy (AFM) and fiber quality analyzer (FQA). It was found from the images of SEM that the original fiber surface was smoothly but markedly corroded after the cellulase modification. Moreover, the modification brought about more external fibrillation and longitudinal tearing during refining. AFM measurement results showed that the fiber surface roughness was improved after the cellulase modification because that the middle lamella, primary wall and outer layer of secondary wall were broken. The results of FQA measurement showed that the average fiber length increased slightly.