Abstract: Sewage sludge pits exist in many landfills of municipal solid wastes (MSWs) in China, and their safety is particularly important. The failure of a sludge pit and the downstream waste slope during the waste filling over the pit is introduced. The presentation includes many failure evidences, contingency measures, field monitoring of leachate level, surface and deep horizontal displacements before and after the accident. Back analyses are also carried out to have the effect of the sludge pit on the stability of the downstream waste slope. The field monitoring and back analyses demonstrate that the surcharge loading of MSWs over the super-soft sludge pit significantly lowers the factor of safety of the downstream waste slope. The associated mechanism includes two aspects: firstly, the fluid-like sludge transfers the vertical loading of waste into a lateral pressure of the same magnitude acting on the downstream waste slope. Secondly, some fluid-like sludge is squeezed into the surrounding waste body as well as the interface between the waste and the underlying liner, leading to a decrease in the frictional strength. The high pressures of leachate and gas existing in the landfill are the internal and major factor for the failure. Drawdown of the high leachate level increases the factor of safety of the waste slope and prevents a further sliding of the slope. The risk of failure during the filling of MSWs will be significantly reduced if the soft sludge in the pit is firstly improved to a sufficient strength. Field monitoring of horizontal displacement on the waste body surrounding the sludge pit will provide an early warning of the accident.