The invention provides a triaxial test device for in-situ generation and decomposition of a natural gas hydrate. The device comprises a closed pressure chamber consisting of an outer cover barrel and a base; an inner cover barrel is arranged in the closed pressure chamber; the inner cover barrel is fixed on the base through a clamping ring; at least one oil inlet and outlet hole, at least one airinlet hole and at least one water inlet hole are formed on the base of the inner cover barrel; the oil inlet and outlet hole, the air inlet hole and the water inlet hole are formed on the inner cavity of the inner cover barrel; pistons are arranged on the tops of the outer cover barrel and the inner cover barrel; air exhaust holes are formed on the pistons; a lower pressure pad and an upper pressure pad which correspond to each other are arranged in the inner cavity of the inner cover barrel; the upper pressure pad corresponds to the positions of the pistons; and the lower pressure pad is fixed on the base.