The invention discloses a method for mining sylvite in underground carnallite ore, which comprises the following steps of: (1) building wells, namely, taking three-pipe structures as well bodies, wherein the three pipes include a technical sleeve pipe, a production sleeve pipe and a central pipe; (2) building a groove on a single well, namely, filling normal-temperature water in annular space of the central pipe and the production sleeve pipe, and discharging dissolved brine from the central pipe; (3) mining by non selective dissolution at normal temperature; (4) selectively mining by hot melting at high temperature, namely, communicating two wells or well groups, pulling out the central pipe, filling a sodium chloride and magnesium chloride containing filler of 75 to 90 degrees centigrade into a well, and discharging the brine for processing from the other well in the well group; and (5) refilling a mother solution into the wells. The method for mining the sylvite in the underground carnallite ore provided by the invention realizes selective mining of potassium chloride; and, the method saves energy resources and reduces production cost; moreover, the refilling process of the mother solution realizes zero discharge of the mother solution and facilitates environment protection.