Abstract: Effects of chitosan on photosynthesis of seven kinds of common submersed plants, including Myriophyllum Spicatum,Potamogeton Malaianus,Potamogeton Maackianus,Hydrilla Verticillata,Potamogeton Perfoliatus,Ceratophyllun Demersu and Najas Japonica,were studied by using Iodometric method.Results showed that the total and net photosynthetic rate of common submersed plants can be promoted by pretreatment of appropriate mass concentration of chitosan.The appropriate pretreatment for Potamogeton Perfoliatus,Potamogeton Malaianus,Myriophyllum Spicatum,Potamogeton Maackianus and Hydrilla Verticillata was 1.0 g/L chitosan for 24 hours,and for Najas Japonica and Ceratophyllun Demersu,it was 2.0 g/L chitosan for 24 hours.Species and specific surface area of submersed plants were two key factors to decide the appropriate concentration of chitosan.Photosynthesis ability and growth of submersed plant could be promoted by appropriate concentration of chitosan.It could be applied in the lake ecological restoration.