Abstract: The horizontal trench is a major leachate drainage facility in municipal solid waste (MSW) landfills, of which the spacing design affects drainage and water level greatly. Based on a new seepage model for horizontal trench, the analytical solution of saturation line and the relationship between design spacing of horizontal trench and warning water level can be deduced. The results of numerical calculation agree well with the analytical solutions. It is further revealed that how the inflow, saturated hydraulic conductivity of MSW, size and spacing and equivalent hydraulic conductivity of the horizontal trench influence the capacity of water level control. The results show that the maximum head of waste body and the head at horizontal trench significantly increase when the hydraulic conductivity of horizontal trench drops below 1×10^-4cm/s; and it is in favor of the control of water level when the ratio of the inflow to the saturated hydraulic conductivity of MSW is less than 5×10^-2. A dimensionless figure of spacing design of horizontal trench is proposed, and is verified by a typical case analysis.