The invention discloses a method for measuring methanesulfonate in methanesulfonic acid by a derivatization HPLC-UV method. The method uses a dithiocarbamates derivatization reagent for derivatization of methanesulfonate to generate product having strong absorption capacity in an ultraviolet light zone; a derivative reaction liquid is taken as a sample introduction sample, based on an anti-phase high-performance liquid chromatography principle, the derivatization products of the methanesulfonate are separated and determined, so that qualitative or quantitative determination can be carried out on the methanesulfonate. According to the invention, the methanesulfonate and the dithiocarbamates derivatization reagent are subjected to a reaction, the generate product has UV absorption characteristic, and the invention establishes the simple and general method for measuring methanesulfonate in methanesulfonic acid by the derivatization HPLC-UV method. The method has good specificity and good sensitivity through results by methodology validation.