The invention discloses an online detection device for temperature and downward-press amount in a friction stir welding process. The online detection device comprises a circular fixture, a frame typesupport, an adjusting assembly, a temperature detection module, a downward-press amount detection module and an industrial personal computer, wherein the circular fixture, the frame type support and the adjusting assembly are mounted on a friction stir welding main shaft, the temperature detection module and the downward-press amount detection module are mounted on the adjusting assembly, the industrial personal computer is in communication connection through data wires, the circular fixture comprises semicircular fixtures, an adjusting fixing device and a silica gel pad, and is located abovethe frame type support, and the adjusting assembly comprises guide rails, sliding blocks, a fixed shaft, a rotating shaft, an adjusting block, a shaft seat, a shaft sleeve and a sensor support. According to the detection device, through the frame type support, an infrared thermal imager and a laser ranging sensor move along with a stirring needle in the friction stir welding process, so that the effect of being influenced by a welding base material and a welding process is avoided, and the temperature and the downward-press amount of the friction stir welding process can be detected online sothat the welding state can be conveniently analyzed in time, and the welding quality can be ensured.