The invention relates to a method for fuel oil extraction desulfurization by polyethyleneimine. A polyethyleneimine extracting agent with the average molecular weight being 600 to 10000 and fuel oil are mixed and extracted according to a mass ratio being 0.2 to 3; 10 percent to 80 percent of organic sulfur in the fuel oil can be removed; and the oil product recovery rate is high; the selected extracting agent is not dissolved in the fuel oil; the oil pollution cannot be caused; the extracting agent cannot be lost; the regenerated cyclic utilization potential is realized. For the whole desulfurization process, the operation conditions are mild; high-temperature and high-voltages conditions are not needed; the operation is simple; the required equipment is not complicated, so that the desulfurization cost is low; the method is applicable to desulfurization treatment on various kinds of fuel oil such as high-boiling-range gasoline, aviation kerosene, vacuum gas oil and diesel oil.