The photodegradation of clopyralid in pure and surface water under UV and simulated sunlight irradiation, were investigated to guide the treatment of clopyralid contaminated water in a natural environment. Under UV irradiation in pure water, the degradation rate of clopyralid was promoted with the decreasing of initial concentration and the degradation followed pseudo-first-order kinetics. Because of the competition between clopyralid and dissolved organic matters (DOM) for UV adsorption in surface water, the clopyralid degradation rate was lower in surface water than in pure water. The addition of H2O2 significantly enhanced the UV photodegradation of clopyralid; for example, after the addition of 0.2 mM H2O2 and 240 min irradiation nearly 100% degradation was achieved, compared with only 15% degradation without H2O2. To further understand the degradation process of clopyralid, the probable degradation pathway of clopyralid under UV/H2O2 was proposed. The removal of clopyralid was more effective under acidic condition than under neutral and alkaline conditions. Under simulated sunlight irradiation, there was no degradation of clopyralid in pure water, while it could be removed by adding hematoporphyrindihydrochloride (HPDHC) through indirect photodegradation, as HPDHC could generate single oxygen by absorbing the long wavelength sunlight and the single oxygen could facilitate clopyralid degradation. Similarly, a slight decomposition was observed in surface water, which was induced by DOM.