The photodegradation mechanisms of nonylphenol (NP) with the involvement of dissolved oxygen (DO), hydrogen peroxide, nitrate ion and chloride ion in water by simulated sunlight were studied, respectively. The influence of each factor on the photodegradation of NP has been assessed through contrasting the residual rate of NP. And the photodegradation pathways in different conditions have been proposed according to the intermediate products identified by GC-MS. DO was a key factor in the photodegradation of NP and the rate of photoreaction depended on O₂ concentration; the existence of H₂O₂ or NO₃^- increased the NP degradation rate significantly; with the existence of H₂O₂, the addition of Cl^- first accelerated then slowed down the NP degradation rate. When NP reacted with DO, 4-nonylcatechol, nonanol, nonanal and nonoic acid have been identified as the degradation products. The accumulated amount of nonoic acid was the most. The proposed mechanism was that the 4-nonylcatechol and ortho-quinone derivative were produced after the formation of 4-nonylphenoxyl radical and superoxide radical anions (O₂^·-), then the intermediates underwent conjugate addition, nonanol, nonanal and nonoic acid were produced. In the presence of H₂O₂, 4-n-akylphenol (HOC₆H₄-C_nH_2n+1, n＝2～8), 4-nonylcatechol, nonanol, nonanal and nonoic acid were the degradation products. And the accumulated amount of nonanal was the most. The hydroxyl radicals (·OH) generated by the photolysis of H₂O₂ attacked the electronic gathered positions of NP molecules. The products detected in the presence of NO₃^- were same as which detected in the presence of H₂O₂. The irradiated NO₃^- can produce ·OH and ·NO₂, so 2-nitryl-4-nonylphenol was found as the product of ·NO₂ attacking the ortho-position of phenolic hydroxyl. In the presence of Cl^- and H₂O₂, the nonanoyl chloride was found. The accumulated amount of nonanal was the most, so the probable mechanism was that the chlorine radical which generated from the reaction of ·OH and Cl^- reacted with nonanal.