The invention relates to a dosing and size mixing method for the ore dressing of high-carbonate and low-grade phosphorite, in particular to a method for adding an agent in the ore dressing process of removing carbonate from the high-carbonate and low-grade phosphorite. The method comprises the following steps of: adding a phosphoric acid or phosphate inhibitor; adding a sulfuric acid regulator; and adding a collecting agent at the position which forms an angle of between 90 and 180 degrees with an inlet of ore pulp and the position 10 to 20 centimeters higher than an ore feed port in the rotating direction of the ore pulp in a stirring barrel while adding sulfuric acid into the stirring barrel. By the dosing and size mixing method, carbonate minerals in the phosphorite can be removed effectively; calcium and magnesium ions generated by reacting the carbonate and the sulfuric acid can be combined with fatty acid ions quickly, so that the generation of indissolvable calcium sulfate and magnesium sulfate is prevented effectively; and even if crystalline solids of the calcium sulfate and the magnesium sulfate are generated, the crystalline solids react with fatty acid due to more active points on the surfaces to prevent the crystalline solids of the calcium sulfate and the magnesium sulfate from depositing on the walls of equipment or pipelines, so that the condition of blockage of the ore dressing equipment and the pipelines due to the deposition of the crystalline solids of the calcium sulfate and the magnesium sulfate is relieved.