In order to resolve problems such as the obsolescence of equipment, the backwardness of technology, and the lack of assurance regarding the quality of filling slurries, a case study is conducted on the technical transformation and capacity expansion project of a domestic in-production metal mine. The transformation process of the existing filling preparation system in this mine is introduced. Through a comprehensive comparative analysis of tailings thickening methods, slurry mixing approaches, and slurry conveying means, a transformation plan for the filling preparation process is put forward. Moreover, calculations and model selections are performed for the filling capacity, material balance, and main process equipment. Practical applications have demonstrated that the filling process incorporating a deep-cone thickener can effectively stabilize the underflow concentration, enabling steady-state filling. Additionally, the solid-content ratio of the overflow water is relatively low. The research findings of this paper can offer substantial technical-solution support for the implementation of the technical transformation and capacity-expansion project of the mine. Meanwhile, they can also serve as a reference for the selection of filling-preparation processes in similar domestic mines.