Disclosed is a method of constructing partially earth-anchored cable-stayed bridges using a thermal prestressing technique, in which, when a steel girder-type partially earth-anchored cable-stayed bridge is built using a cantilever construction technique, the center of an intermediate span of the bridge is closed with a final key segment using a thermal prestressing technique, thus applying an initial axial tensile force to reinforcing girders of the bridge. To apply the initial axial tensile force to the reinforcing girders while the center of the intermediate span is closed with the final key segment, an appropriate space length required for closure of the final key segment is determined, and both the heating region and the heating temperature of the reinforcing girders according to the initial axial tensile force to be applied to the reinforcing girders during a process of manufacturing the final key segment are determined. Thereafter, the reinforcing girders are heated using a heating means according to the above-determined conditions, thus being thermally lengthened to predetermined lengths corresponding to the predetermined space length. The junction between the reinforcing girders at the center of the intermediate span is closed with the final key segment and, thereafter, the heating means is removed from the reinforcing girders.