The present invention relates to an improvement on the process to prepare potassium chlorate, in which potassium chlorate may be obtained by one-pass crystallization after the potassium chloride liguor and sodium chloride liguor are mixed or the solution of potassium chloride is electrolyzed. Additive such as anionic surfactant or ampholytic surfactant with anions is added into electrolyzed liquor whose components include KClO3 (200-230 g/l), KCl (20-40 g/l), NaCl (0-200 g/l) and Na2Cr2O7 (3-5 g/l) and whose temp is 70-100 deg.C while the content of additive is controlled to 1-10 ppm. Through uniform mixing, crystallization, solid-liquid separation, baking and breaking, potassium chlorate product is produced out.