[Objective] To develop the clinic effectiveness of arthroscopic debridement with medical chitosan for moderate and severe osteoarthritis of the knee. [Methods] From Sep 2014 to Sep 2019, we treated 248 unilateral moderate-severe osteoarthritis of the knee, who were divided to surgery group and conservative group. Surgery group were treated by arthroscopic debridement with medical chitosan, and conservative group were only treated by medical chitosan. Before treated and 6weeks, 12 weeks ,24 weeks ,48weeks after that, therapeutic evaluation and index of severity for osteoarthritis(ISOA) and Lysholm scores were assessed. [Results] The good results were got in two groups. The effective rate of the surgery group were better（t=5.265 P＜0.05） than that of the conservative group. The ISOA and Lysholm scores of the conservative group decreased gradually, otherwise that of surgery group can maintained. [Conclusion] Arthroscopic debridement with medical chitosan for moderate and severe osteoarthritis of the knee is effective in a middle or short term, and it can be used widely.