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.