It is usually reported that chitosan is used as paper strengthening agent. However, it is sorry that the study choosing chitosan which DD (degree of de-acetyl )and viscosity is filtered by experiments is blank. Many chitosan samples of wide viscosity range and wide DD range were tested by their strengthening effect on bleched wheat straw pulp. Then a few sample of chitosan is selected because of their obvious strengthening effect on pulp. This study could be as the foundation of further research of chitosan used as paper strengthening agent.