The invention discloses a water-cooled microwave ablation needle. A first annular cavity is formed between a shielding needle tube and a microwave coaxial cable, a second annular cavity is formed among the cylindrical back end of an ablation needle head, an insulator and the shielding needle tube, and the first annular cavity is isolated from the second annular cavity through a sealing ring. According to the water-cooled microwave ablation needle, circulating cooling water is led into the first annular cavity and used for reducing the temperature surrounding the microwave coaxial cable, a dual-metal temperature sensing element and a cooling-water-leading capillary tube are arranged in the second annular cavity, the spirally-curly dual-metal temperature sensing element is wound or unwound along with changes of temperature to drive a transmission rod to rotate in a through hole, a cover plate is moved away from an opening of the capillary tube or covers the opening accordingly, the flow of the cooling water in the capillary tube is increased or reduced accordingly, and carbonization, caused by excessively high temperature, of tissue is avoided on the premise that the ablation range is not influenced.