The invention relates to a less slag decarbonization refining method used in a top-bottom combined blowing converter. The invention belongs to the technical field of top-bottom combined blowing converter steelmaking. A steelmaking slag amount is strictly controlled, a slagging technology is modified, high-alkalinity slag with R no less than 3.5 is formed as quickly as possible, technological parameters such as steel carbon content and end point temperature are appropriately controlled, a phosphorus distribution ratio LP among the slag steel is no smaller than 200, a melting pool dephosphorization rate is maintained at a value no less than 80%, such that low-phosphorus steel with [P] no larger than 0.008% is produced. Calculated according to phosphorus balance, when semi-steel [P] is no larger than 0.03%, and end point [P] is no larger than 0.008%, the dephosphorization rate during the steelmaking procedure is no larger than 0.022%, such that the P2O5 amount is 0.5kg/t. If the slag amount is controlled at a value no larger than 30kg/t, the P2O5 content in the slag is no larger than 1.6%. When the phosphorus distribution ratio LP among the slag steel is no smaller than 200, in the steel balanced with slag, [P] is no larger than 0.008%. The method is suitable to be used in decarbonization refining of molten iron pretreated by a three-removing process in the top-bottom combined blowing converter. The method is advantaged in that: through the improvement of the converter availability, an oxygen supplying time is controlled in 11min, a smelting period is no larger than 25min, a daily smelting amount is no less than 40 times/furnace, such that a problem of insufficient decarbonization furnace heat is solved.