A solid-liquid separation device includes a kettle body, a piston, a stirrer, a separation plate having filtration pores, and a filtration mesh. The kettle body is hollow along an axial direction to form a chamber body. The separation plate is fitly installed in the chamber body, and divides the chamber body into a washing chamber and a draining chamber. The piston and the stirrer are fitly disposed in the washing chamber. The filtration mesh is attached on a side of the separation plate to cover the filtration pores. The kettle body is further provided with a feed inlet, a water inlet, a material outlet, and a liquid outlet. The feed inlet and material outlet are communicated with the washing chamber, and the water inlet and the liquid outlet are communicated with the draining chamber. The method includes the following steps: feeding, solid-liquid separation, washing, and material discharge.