The invention relates to a high-voltage large-section cable thermal displacement monitoring device, which comprises a first displacement sensor, a second displacement sensor and a data acquisition module, and is characterized in that the first displacement sensor is arranged at the lowest part of a bent cable between adjacent hoops, an auxiliary base matched with the first displacement sensor is arranged under the first displacement sensor, and the top surface of the auxiliary base is horizontally arranged; the second displacement sensor is arranged at cable surface close to the hoop, and theside, which faces towards the second displacement sensor, of the hoop is provided with a reflecting surface perpendicular to the horizontal plane; and both the first displacement sensor and the seconddisplacement sensor are connected with the data acquisition module, and the data acquisition module is connected with a display module. The device facilitates the operation and maintenance personnelto check axial and amplitude displacement of a cable and can discover an exception in advance to avoid occurrence or expansion of a fault.