Water changes inside loess during the formation and dissociation processes of methane hydrate were studied with a pF-meter sensor which could detect water changes inside soil. It was proved that the new research method with pF-meter sensor could instantly detect the water changes inside loess in the experiment, effectively. It was experimentally shown that hydrate formation could cause a kind of suction which could pump the water inside loess toward hydrate and the water release from hydrate dissociation could increase water content of loess and the water existing in the capillaries of loess could flow down following temperature rise. Additionally, the scope of secondary methane hydrate formation in loess was larger than that of the initial hydrate formation.