Provided are an all-solid-state battery and a preparation method thereof. The all-solid-state battery includes a positive electrode, a negative electrode, and a solid-state electrolyte located between the positive electrode and the negative electrode. The negative electrode includes a first negative electrode and a second negative electrode. The second negative electrode is located on a side of the first negative electrode. The solid-state electrolyte includes a first solid-state electrolyte and a second solid-state electrolyte. The first solid-state electrolyte is located between the positive electrode and the first negative electrode. The second solid-state electrolyte is located between the positive electrode and the second negative electrode. The roughness of the second solid-state electrolyte is greater than the roughness of the first solid-state electrolyte.