The invention provides a diesel oil blending agent for an agricultural machine, which is prepared from a product obtained by hydrogenating and separating a byproduct C10 fraction of an ethylene unit, an ash-free dispersant polyisobutylene succinimide and 0# diesel oil, wherein the sum of the weight of the components is 100; and the product, the ash-free dispersant polyisobutylene succinimide and the 0# diesel oil are 65-75%, 0.2-0.3% and 24.8-34.7% of the total weight in sequence. The byproduct C10 fraction of the ethylene unit is characterized in that: the boiling range is 200-280 DEG C, thedensity (at 20 DEG C) is 0.8-0.86 kg/m3, the kinematic viscosity (at 20 DEG C) is 1.75-2.0 mm2/s and the freezing point is below 30 DEG C. By utilizing a plurality of agricultural tractors to be examined, the oil consumption of each kilogram is improved by 2-4% along the increasing of the adding amount of the diesel oil blending agent, however, the total fuel cost is reduced by about 10%; the carbon content in the tail gas can be reduced by 15-25%; and an anti-freezing test shows that the 0# diesel oil, -10# diesel oil and -20# diesel oil can be respectively utilized at the temperatures of below 10 DEG C, below 20 DEG C and below 25 DEG C.