The invention provides a converter dephosphorizing and steelmaking method. The method comprises the following steps of: a, adding slag making materials which consist of active lime, high-magnesium lime and a composite slagging agent into a converter molten pool; blowing and slagging by a dephosphorizing oxygen lance oxygen blowing process and an inert gas bottom blowing process to remove phosphorus in molten steel; and pouring 60 to 80 percent of slag out when the molten pool temperature in a converter is between 1,420 and 1,450 DEG C, the alkalinity of the slag is between 2.0 and 2.5 and the total iron content in the slag is between 10 and 15 weight percent; and b, adding the slag making materials into the converter molten pool; blowing and slagging by a normal oxygen lance oxygen blowing process and the inert gas bottom blowing process to further remove the phosphorus in the molten steel; pouring the slag out when the molten pool temperature in the converter is between 1,680 and 1,700 DEG C, the alkalinity of the slag is between 3.2 and 4.2 and the total iron content in the slag is between 16 and 23 weight percent to obtain the molten steel with the phosphorus element content of not more than 0.009 weight percent. By the method, the dephosphorizing efficiency and the dephosphorizing effect of the converter are improved, and the production cost is reduced.