Disclosed is a system for monitoring methane dissolved in an underground water body in a frozen earth region for a long time. Gradient-type in-situ dynamic monitoring conducted on water-soluble methane in a natural gas hydrate stabilized zone under a frozen earth layer is achieved, the problems of difficult in-situ collection of dissolved-methane-containing pore water under the frozen earth layer, water body in-situ state change caused by frozen earth ice layer de-icing and the like are solved, the change of the soluble methane content in the natural gas hydrate stabilized zone is really reflected, and basic parameters are provided for research on the scientific problems such as hydrate existing condition change, methane dissolution and methane migration in a frozen earth zone environment. The system comprises a sensor integration unit, a transmission cable, a control system, an acquisition system, a remote control unit and a power supply unit. The invention also provides a method for monitoring methane dissolved in the underground water body in the frozen earth region for a long time.