The invention discloses a device and a method for dehydrating heavy sump oil, which belongs to the technical field of petrochemical wastewater treatment. The method has the characterized by comprising the following steps of leading out sump oil in a tank area and entering the sump oil into a Y-shaped filter; filtering, boosting via a booster pump, and then entering the sump oil into a steam heat exchanger (1), and heating to 80-130 DEG C; then introducing the sump oil in a vortex dehydrator (2), and separating the sump oil into a low-water-content sump oil phase and a low-oil-content sewage phase, and entering the low-water-content sump oil phase into a coalescence dehydrator (4), further dehydrating to obtain low-water-content oil, and leading the low-water-content oil out of the device, and meanwhile, entering the low-oil-content sewage phase into a vortex oil remover (3), separating the low-oil-content sewage phase into a high-water-content sump oil phase and a sewage phase, discharging the sewage phase, and cyclically performing a dehydration treatment on the high-water-content sump oil phase. According to the device and the method, the continuous performance of dehydration for heavy sump oil is realized, deep dehydration and deep oil removal are performed simultaneously, and non-thorough parts can be cyclically dehydrated and separated; the device and the method are low in energy consumption, convenient to operate, high in dehydration rate, strong in adaptability to heavy sump oil with various water contents and dirt contents, and suitable for performing a long-term treatment on a large amount of heavy sump oil.