Some sanitary landfills in China are required to treat aging landfill leachate that is highly saline. In this study, the effectiveness of an emergency disk tube-reverse osmosis (DTRO) treatment system for such a refractory mature landfill leachate was evaluated. A molecular-level analysis was then applied to reveal the changes of the characteristics of leachate organic matter (OM). The DTRO system achieved > 83% water recovery rate, reduced the electrical conductivity of effluent to 0.15–0.22 ms/cm, and reduced carbonaceous and nitrogenous pollutants to a level suitable for discharge. Furthermore, the vast majority of salts (e.g., chloride and sulfate ions), as well as refractory OM (e.g., humic- and fulvic-like substances), were effectively removed. The DTRO system can effectively remove a great number of macromolecular dissolved organic compounds with carbon number > 23, as well as highly unsaturated compounds with DBE > 12. Additionally, > 80% of the formulas assigned for the dissolved OM (DOM) was removed; even CHONS compounds with complex molecular structures were completely removed. The constitution of DOM in the DTRO effluent was simple, mainly comprising anti-sludge agents (e.g., small molecule alcohol and alkyl benzene sulfonic acid, etc.). However, the DOM in the resulting membrane concentrates was very similar to that in raw landfill leachate and the concentration was much higher. Therefore, an effective and feasible method should be developed to treat DTRO membrane concentrates because they pose high environmental risk.