We proposed and analyzed the D-P (Dye-to-Photon) photobleaching mechanism, the interaction between the excitation photon(s) and the fluorophore dye molecules in excited states. At single molecule level, the D-P photobleaching probability with One-and Two-photon excitation (OPE and TPE) should always depend linearly on the probability of fluorophore molecule in excited states. However, at biological imaging level, the dependence of the D-P photobleaching probability on the probability of fluorophore molecules in excited states may depend on the excitation way: the photobleaching rate with OPE should always depend linearly on the probability of fluorophore molecule in excited states, while the photobleaching rate depends on the high-order power of the probability of fluorophore molecule in the excited states, especially in the case of high excitation intensity.