A foundation treatment method for piling a foundation structure by penetrating a hardpan layer, i.e., a Deep Slurry Mixing method, comprising following steps: disturbing, by a mechanical device, a location where the foundation structure is to be piled, so that the mechanical device penetrates the hardpan layer of a natural foundation; then injecting clay slurry into the hardpan layer of the natural foundation by a pumping device, an improved foundation is formed after mixing; and piling the foundation structure. The method can change soil property of the original natural foundation, break the hardpan layer, reduce piling resistance of the steel plate cylinder or similar foundation structure, reduce uneven force during the piling process and improve driveability.