Abstract: Mature Vallisneria natans were planted in 30 NTU,60 NTU and 90 NTU turbid waters made by suspend sands with less than 100 μm,respectively.After one month,the longest leaf under 10 cm water were measured by a developed,submersible,pulse-amplitude modulated (PAM) fluorometer,and diving-PAM,and diurnal variation of ΔFv/Fm and rapid-light curves(RLC) were also measured.The results indicated that the turbid water had not significant influence on Fv/Fm,which indicated that photosystem II of V.natans hadn’t been influenced apparently.The diurnal variation of V.natans’s ΔF_v/F’_m in irradiance looked like "V" shape.When turbidity of water was more than 60 NTU,ΔF_v/F’_m of these plants decreased slightly,and restored quickly as irradiance became small.The results of RLCs suggested that V.natans began caducity,and their light responding ability of leaves decreased during the experimental period.With turbidity of water increasing,light responding ability of their leaves decreased slowly.All results indicated that the water turbidity had little influence on photosystem Ⅱ of these plants during the experiment.