We describe here the biodegradable polymers consisted of the tandemly linked phosphate-caged fluorescein in the main chains. The caged-fluorescein monomer with the phosphoramidite groups at both hydroxyl groups was synthesized, and the polycondensations were executed in the presence of the diol derivatives which were promised to be the reactivity-tunable linker. After oxidation and the alkaline treatment for the deprotection, less-emissive polymeric products were obtained. The resulting polymers can release fluorescein by the digestion with alkaline phosphatase. We demonstrated that the enzymatic activity of the cell lysate can be quantified from the increase of the magnitude of fluorescence emission with the synthetic polymers.