The invention discloses an ethylene cracking and cleaning agent and a preparation method thereof. The ethylene cracking and cleaning agent is prepared from Tween 80, citric acid, sodium persulfate, potassium permanganate and secondary deionized water, wherein the percentage by weight of each component is as follows: the Tween 80 accounts for 15-25%, the citric acid accounts for 0.4-0.6%, the sodium persulfate accounts for 15-25%, cinnamaldehyde accounts for 4-6%, the potassium permanganate accounts for 1-2%, and the secondary deionized water accounts for 40-50%. The ethylene cracking and cleaning agent disclosed by the invention has the beneficial effects of high efficiency, no toxicity, no corrosion, no secondary pollution and fast passivation action against ferrous sulfide, and can effectively prevent spontaneous combustion of the ferrous sulfide; the using method is simple, large process adjustment is not required, and only small alteration is needed on the basis of an existing device; the ethylene cracking and cleaning time is 8-10 hours, more than 90% of the ferrous sulfide can be removed, and the shutdown time of the device is correspondingly reduced; a protective film can be formed on the surface of metal after cleaning, so that the surface of the metal can be effectively prevented from being further corroded; and a mixed solution after being treated by the cleaning agent can be directly discharged without the impact on a sewage treatment field.