In this work, a new technique of modified-alkali leaching-dilute acid washing for extracting synthetic rutile from high-titanium slag is proposed. Most of the Ti components are enriched into the rutile phase during the modification, and rutile can be separated by alkali leaching-dilute acid washing. Thermodynamic analyses of the reactions occurring during the alkali leaching-dilute acid washing process were performed to provide a theoretical basis for rutile extraction from the modified slag. The calculated thermodynamic results suggest that it is possible to use the alkali leaching-acid washing process to separate rutile from the impurities in the modified slag. The structure of the glass phase, which contains almost all of the impurities, requires relatively high alkalinity to be destroyed, and the impurities of the slag, including Fe2O3, CaO, MgO, and MnO, can be removed easily with dilute acid after the alkali leaching stage. The technique is low in cost and obtains synthetic rutile for the chlorination process.