The invention belongs to the technical field of the material science, and particularly relates to an online detection device for variation of a metal melt structure caused by the ultrasonic preprocessing. The detection device comprises a heating heat preserving system, a metal melt storage unit, a power ultrasonic system, a melt resistor or a resistivity detection system, a melt temperature signal collection system, a data collection and processing system and the like. By adopting the detection device, the metal melt structure variation caused by the ultrasonic can be detected and represented by utilizing a variable quantity delta r of physical parameters-resistance parameters of structure sensitivity, so that the insulation problem of a power ultrasonic lead-in pole can be successfully solved, the influence on an ultrasonic bar caused by utilizing a voltammetry to measure the resistance of ultrasonic metal melt can be overcome, the real-time, synchronous, continuous and high-precision automatic collection and processing of the resistance and temperature data can be realized by utilizing the advanced electronic measurement technology and computer technology, and the precise measurement can be realized by adopting a double-electrode method and a four-electrode method.