A schiff-base dual emission ratiometric fluorescent chemosensor which includes coumarin dye and 5-aminoisophthalic acid methyl ester has been designed and synthesized. The chemosensor CA high selectively detects Hg2+ ions in the presence of other biologically important and environmentally relevant metal ions at physiological pH. The double bond of schiff-base was broken accompanied by fluorescence enhancement when the chemosensor interacts with Hg2+ ions, and the coumarin aldehyde and 5-aminoisophthalic acid methyl ester shows strong ratiometric fluorescent emission respectively. And the chemosensor could be applied in cellular imaging at nano molarity by dual fluorescent emission.