Sulfur was removed from diesel oil by using the ultrasonic oxidation process to reduce the sulfur content in diesel fuel. The effect of different oxidation temperature, oxidation time, oxidant dosage and catalyst dosage on diesel desulfurization were studied by experiments. Experimental results showed that using the formic acid mixture with sulfuric acid as catalyst, catalyst amount 2% (the volume ratio of formic acid and sulfuric acid in the catalyst was 3∶2), oxidant amount 9%, reaction temperature 70 ℃, reaction time 60 min, removing the sulfur in heavy oil catalytic cracking diesel oil by using ultrasonic oxidation, then through DMF extraction and oxidation, the diesel oil desulfurization rate could reach 83%， And cetane value was increasing, thus the quality of diesel oil was refined.