In the current study,the nano Ag sol of grass carp scale collagen(Col-nAg sol) was prepared by chemical reduction method,using silver nitrate solution with tea polyphenols(TP) as reducing agent and while collagen of grass carp scale as stabilizer.Effects of AgNO3 concentration,TP concentration and pH value on the preparation of Col-nAg sol were studied.Results showed that the optimal preparation conditions for Col-nAg sol were as follows:0.02 mol/L for AgNO3,pH 7.0 and 2 mol/L for TP.The average particle size for nano-Ag was 4.45 nm,which could be preserved at 4 ℃ for 60 days without obvious change.The Col-nAg sol was applied in the detection of Cr3+.Non-crosslinking agglomeration occurred between the nano-Ag particles and Cr3+,which caused a dramatic color change of Col-nAg sol from yellow to brown and a larger particle size of nano silver.The method was highly selective and sensitive in the detection of Cr3+ with a detection range of 0-10 μg/mL and a detection limit of 0.5 μg/mL.Parameters such as detection time,temperature and pH were optimized to obtain precise detection limit.