Embodiments of mechanisms for forming a metal-insulator-metal (MIM) capacitor structure are provided. The metal-insulator-metal capacitor structure includes a substrate. The MIM capacitor structure also includes a CBM layer formed on the substrate, and the CBM layer includes a bottom barrier layer, a main metal layer and a top barrier layer. The MIM capacitor structure further includes a first high-k dielectric layer formed on the CBM layer, an insulating layer formed on the first high-k dielectric layer and a second high-k dielectric layer formed on the insulating layer. The MIM capacitor structure also includes a CTM layer formed on the second high-k dielectric layer, and the CBM layer includes a bottom barrier layer, a main metal layer and a top barrier layer.