Methods for treating landfill leachate taught herein comprise: (a) coagulatively settling leachate; (b) filtering the resultant supernatant liquid through multistage cartridges; (c) filtering the filtered liquid by reverse osmosis; and (d) treating the resultant liquid with chlorine-based oxidant. These methods provide the advantages of efficient wastewater treatment, compact equipment size, short process flow, and low startup and operational costs.