The demulsifier applicable to emulsification of diesel oil alkaline residue is characterized by that in the gasoline alkaline residue the sulfuric acid is added to make neutralization reaction to obtain waste salt water, and its pH value is controlled at 4-7. Its use method includes the following steps: in emulfified diesel oil alkaline residue adding 10-50 wt% of demulsifier, fully mixing them at 30-80 deg.C, standing still for 1-10 hr emulsion-breaking and demixing, oil and water clear separation and separating waste alkaline water and diesel oil. Said invention uses gasoline alkaline residue as raw material to make acid-alkali neutralization to obtain demulsifier, and uses said demulsifier to make demulsification of emulsified diesel oil alkaline residue so as to can obtain good demulsification effect.