The Invention relates to a method to utilize the characteristic of a protective agent to incubate singlet oxygen and prolong its lifetime so as to provide abundant singlet oxygen with persistence and thus improve the effect of photodynamic therapy. The composition is composed of a functional substance that can prolong the lifetime of singlet oxygen, an emulsifier and a photosensitizer； the functional substance is emulsified into an emulsion through the emulsifier and delivered into a tumor tissue with the photosensitizer； then the photodynamic therapy can be conducted with the abundant singlet oxygen incubated by the functional substance that can prolong the lifetime of singlet oxygen and thus the effect of photodynamic therapy is improved.
