Expansion of municipal solid waste (MSW) landfill is now under practice in many cities of China. The landfill expansion generally involves a significant increase in landfill slope height, which may trigger waste mass instability. Another potential failure associated with the landfill expansion in China is slippage alone the weak interface within the intermediate liner system, which is sandwiched between the existing and expanded waste masses. In this paper, an attempt has been made to to investigate the major factors influencing the slope stability of landfills, including shear strength of MSW, leachate level and interface shear strength within a composite liner. These issues are discussed based on experimental results and theoretical analysis for the Suzhou landfill, which is under design for vertical and lateral expansion.