A “turn-on-off” type fluorescent probe based on terbium coordination polymers was prepared for the detection of trace mercury ions.The terbium coordination polymers were prepared with terbium ions as luminescence center and guanyl 5 disodium phosphate(GMP） as ligand,which could emit green fluorescence.The intensity of the luminescence was increased with the addition of ct-DNA,and a “turn-on” type of fluorescent probe was formed.Then,the “turn-on-off” type fluorescent probe was formed by quenching the fluorescence with the interaction between the heavy metal mercury ions and ct-DNA and ligand of GMP with the addition of mercury ions.The calibration curve for mercury ions was linear in the range of 50-1 200 μg·L-1 ,with a correlation coefficient(r2） of 0.991 5 and a regression equation of(I0-I）/I0=0.000 2 C+0.010 7.The limit of detection was 1.5 μg·L-1,while the relative standard deviation(RSD） was 2.3%(C=600 μg·L-1,n=5）.The probe was applied to the detection of trace mercury ions in river water with recoveries of 98.1%-99.8%and RSD of 0.28%-1.9%.