The invention discloses a preparing method of an oil field demulsifier. The preparing method adopts ethyleneamine as a raw material, and the oil field demulsifier is prepared by crosslinking modification of the ethyleneamine, ethylene oxide, and propylene epoxide. The method comprises the following steps of: with the ethyleneamine as an initiator, enabling the ethyleneamine to react with propylene oxide and ethylene oxide to prepare an ethyleneamine type polyether demulsifier under the existence of a basic catalyst; carrying out self-crosslinking reaction on the polyether demulsifier A by adopting polycarboxylic acid under the action of a catalyst to prepare a product B; and finally carrying out self-crosslinking reaction on the polyether demulsifier A and the product B in a solvent by adopting toluene diisocynate to prepare a high performance demulsifier as a final product. The demulsifier prepared by the preparing method has the advantages of higher relative molecular weight, favorable dehydration effect and favorable broad spectrum, and can be used for demulsification and dehydration of the crude oil in the oil field. The dehydrated water is clear, and the dosage of the demulsifier is less.