A novel fluorescent switch was constructed based on poly 3-{[ 1-( 2-hydrazino-2-oxoethyl ) piperidin-4-ylidene] methyl}thiophene ( PMTH ) in the presence of Pb2﹢ or S-adenosylmethionine ( SAM ) . The switch turned off in the presence of Pb2﹢owing to complex effect, and the fluorescence intensity of PMTH solution was efficiently quenched by Pb2﹢ ions. Upon adding SAM to the Pb2﹢-PMTH solution, which was stronger Pb2﹢ chelators, it could form more stable SAM-Pb2﹢, the Pb2﹢ ion was displaced from PMTH and the fluorescence of PMTH was recovered. By triggering the turn-on signal of PMTH, a new discrimination ability toward SAM method for the determination of SAM was established. The effects of the interaction on the fluorescence spectral characteristics of these Pb2﹢-PMTH and Pb2﹢-SAM complexes in H2 O-CH3 CH2 OH(4:1, V/V) solution, suitable reaction conditions and influencing factors were investigated. This method offered good sensitivity and selectivity for detecting SAM in the presence of amino acids and metal ions. Under optimum conditions, the concentration of SAM in the range from 1. 0 × 10-8-2. 0 × 10-6 mol/L exhibited a linear relationship with the relative fluorescence intensity. The regression equation was △I = 68. 51﹢72. 32c (μmol/L), the correlation coefficient r=0. 9982. The limit of detection was 8. 72×10-9 mol/L. The system was successfully applied for detecting SAM in human serum, urine and injection samples.