The invention provides an MIM (metal-insulator-metal) capacitor and a preparation method thereof. The capacitor comprises a substrate, a first metal layer formed on the substrate, a dielectric layer covering the surface of the first metal layer, and a second metal layer formed on the surface of the dielectric layer, wherein the first metal layer is formed by continuous arrangement of a plurality of metal particles. The preparation method of the capacitor comprises the following steps: providing the substrate; forming the first metal layer on the substrate, wherein the first metal layer has a structure formed by continuous arrangement of a plurality of metal particles; forming the dielectric layer on the first metal layer; and forming the second metal layer on the dielectric layer. By using the capacitor and preparation method thereof, the invention can effectively increase the capacitor density of the MIM capacitor and has the advantages of simple and convenient process and lower power consumption.