A kind of natural cellulose, bleached pine pulp (BPP), was pretreated by mechanical activation (MA) using a self-designed stirring ball milling. The effect of MA pretreatment on the dissolving capacity and molecular chain structure of BPP were investigated by the determination of alkaline solubility (Sa) and degree of polymerization (DP). In addition, the changes in crystal structure of MA-pretreated BPP with different milling times were qualitatively and quantitatively measured by X-ray diffraction and Fourier transform infrared spectroscopy, and the morphology modification was observed by scanning electron microscopy. It was found that MA significantly increased the Sa and reduced the DP of BPP, contributing to the destruction of inter- and intramolecular hydrogen bonds and macromolecular chains in cellulose. The stable crystal structure of BPP was also remarkably damaged during MA processing, resulting in the variation of surface morphology, the increase of amorphous region ratio and hydrogen bond energy, and the decrease in crystallinity and crystalline size, which efficiently increased the accessibility of natural cellulose and would have positive effects on subsequent treatments. The crystalline form of natural cellulose was not changed by MA, and no new functional groups generated during milling.