The invention relates to a bentazone decolouring and purifying method. The method comprises the following steps: alkali extraction, including taking
isatoic anhydride as a
raw material, performing synthetic reactions, such as amidation, sulfonation and
loop closure, in a
dichloroethane solvent system, and carrying out washing and alkali addition for extraction to obtain an alkali-extracted liquid; decolouring, including, transferring the alkali-extracted liquid to a decolouring kettle, adding a certain amount of
activated carbon according to the bentazone content, raising the temperature to 40-95 DEG C, and performing heat preservation for 1-5 h;
filtration, including after the heat preservation is over, transferring the obtained material to a cooling kettle, performing fast stirring, leading brine in for cooling, performing plate
filtration, allowing the obtained filtered liquid to pass an acidification kettle, and then blowing the plate to dry; acidification, including raising the temperature of the filtered liquid in the acidification kettle to 30-70 DEG C, slowly dropwise adding inorganic acid to the filtered liquid till the pH is less than 4, and allowing a great amount of bentazone to precipitate; and
drying, including putting the precipitated material in a centrifugal
machine for direct spin-
drying, and then
drying bentazone in a vacuum condition with the temperature in a range of 60-100 DEG C to obtain the final product. Through the method, a white bentazone primary
drug is obtained.