The invention discloses a methanation reaction system, a power plant peak regulation system comprising the methanation reaction system and a power plant. The methanation reaction system comprises an electrolytic hydrogen production apparatus, a carbon dioxide supply source and a methanation reaction device, wherein the electrolytic hydrogen production apparatus is connected to a power plant power supply apparatus and used for receiving a peak regulation residual power out putted by the power plant power supply apparatus for preparing hydrogen; the carbon dioxide supply source is used for supplying carbon dioxide; and the methanation reaction device is connected to the electrolytic hydrogen production apparatus and the carbon dioxide supply source and used for preparing methane by adopting hydrogen and carbon dioxide as raw materials. The methanation reaction system provided by the invention can directly consume the peak regulation residual power of the power plant, can indirectly utilize the abandoned air, light, water and nuclear power, and can alleviate the power grid balance and peak-valley difference problem. Since the peak regulation residual power is used for generating the hydrogen, and the methane gas is finally produced, so that the indirect storage of the electric power is realized.