Through repeated freezing and thawing of cell，ammonium sulfate precipitation and hydroxyapatite chromatography，the phycocyanin was extracted from spirulina platensis and its purity could reach up to A620/A280=4.1.UV-visible spectra and fluorescence emission spectrometry were used to study the spectral characterisitic of the protein.The results showed that the absorption peaks of the purified protein were located at 280，360,620 nm and the maximum emission wavelength was 650 nm.Using this phycocyanin as fluorescence probe，a new approach was developed for the sensitive and selective determination of divalent mercury ions based on fluorescence quenching.Different influence factors，including pH value，reaction time，temperature and phycocyanin concentration，were investigated.In 0.05 mol/L KH2PO4-Na2HPO4 ,at a pH value of 7.5，phycocyanin concentration of 3 mg/L，reaction time of 30 min and reaction temperature of 30 ℃，the relative fluorescence intensity decreased linearly with mercury ions concentration in the range of 0.1-10 μmol/L.The detection limit was 0.056 μmol/L.This approach displayed a good selectivity to mercury ions.When concentration ratio of interference ions to mercury ion reached to 40∶1，several kinds of coexistence ions had little influence on the detection of mercury ion.This new method exhibits a bright application prospect in the detection of mercury ions due to its advantages of good simplicity，sensitivity，reproducibility and pro-environment.