In order to study the local stability of slurry trench of diaphragm wall in cohesive soil sandwiched sand, mechanical model was established to analyze the stress state of the sand layer. Based on Rankine limit equilibrium principle, calculating method for local stability coefficient of slurry trench was proposed. The main conclusions through the analysis of example are as follows: (1) excavation will lead to negative pore water pressure in the soil element with sand layer, and the local stability coefficient will gradually decrease with the dissipation of negative pore water pressure. (2) the variation of local stability coefficient with relevant factors was obtained: the stability coefficient decreases about 7% for each 10 kPa increase of ground overload and increases about 13% for each 0.5 kN/m³ increase of mud gravity. It increases about 11.5% for every 3 degree increase of effective internal friction angle. Local stability coefficient increases with the depth of the groundwater level at a decelerated growth rate, while decreases with the increase of slurry liquid level depth at an accelerated growth rate.