The preferential flow of leachate in landfill is unavoidable. Therefore, the mixed leachate formed by mixing young leachate and old leachate is the form of leachate mainly present in the gravel layer. In this study, static anaerobic culture experiments were used to investigate the changes in the properties of the mixed leachate. During the experiment, the chemical oxygen demand (COD) content of the three mixed leachates decreased greatly; CODs of 10%, 20%, and 30% were reduced by 12%, 38%, and 24%, respectively. The concentrations of Ca²⁺ and Mg²⁺ also decreased; Ca²⁺ was significantly reduced after a short time. The pH also dropped. The oxidation-reduction potential (ORP) of the three mixed leachates increased, indicating that the reducing organic matter in the leachate was gradually decomposed by microbial oxidation. Analysis by X-ray diffraction (XRD) showed that the main inorganic precipitate is magnesium-containing calcite, an isomorphous crystal. Studies have shown that if there is a mixed leachate in the gravel layer, severe clogging occurs therein. The clogging material is mainly a magnesium-containing calcite precipitate formed by the metabolic activity of organic matter in the microbial degradation of mixed leachate. Some suggestions were made for the operation and management of landfills.