Sulfonated Phthalimidomethyl Zinc Phthalocyanine is a new photodynamic therapy reagent, it can be selectively attacked to cell, but how it attacks the target cell is still unknowable. The aggregation state of photosensitizer is significant to its mechanism proved in CEL solution. The aggregation of ZnPcS2P2 in CEL has been studied with absorption spectroscopy and fluorescence spectroscopy. The aggregation state is dimer through theory analysis, monomer molar absorption coefficient as well as equilibrium constant were estimated.