In order to investigate a fast preparation method of modified nanocellulose, the polyelectrolyte brush type cellulose nanowhiskers (CNW) were prepared by surface grafting with low molecular weight poly (acrylic acid) (PAA) using the UV photo-radiation method. The grafted products (CNW-g-PAA) were characterized by FTIR, 13C NMR, TEM, XRD, DSC etc. The effects of the grafting polymerization conditions, such as the monomer content, the photo-initiator amount, the graft time, which influenced the carboxyl content of the grafted products, were investigated in details. The optimum grafting condition was that the mass ratio of PAA and CNW was 1:1, the mass ratio of the photo-initiator and CNW contents ranged from 8‰ to 10‰, the graft time was 90 seconds, respectively. The optimum highest grafting ratio of carboxyl groups was about 0.01275 mol/g. This UV-grafting method utilized less chemical reagents, less synthesis steps and shorter reaction time. The synthesized CNW-g-PAA would have a lot of applications.