The submicron cellulosic fiber(SCF) was prepared by acid splitting cotton fiber in H2SO4 solution. Laser particle size analyzer, scanning electron microscopy(SEM) and FTIR spectrometer were used to evaluate the diameter, structure and composition of the SCF. It is found that the natural cellulosic fiber split into more fine fiber, with a diameter range from dozens of micron to hundreds of nanometers, decreasing with the increasing sulfuric acid concentration or prolonging reaction time. Then the effect of SCF on the micro structure of cement paste was investigated by SEM and FTIR. The test results indicate that the compatibility of cellulosic fiber and cement paste is well, the hydration product C S H gel grows on and along the surface of SCF and finally embed it inside. The induction and bridging effect of SCF make the microstructure of cement paste more homogeneous.