The water-in-aging crude oil (WACO) emulsions treatment is an urgent problem in the offshore platform because of the low demulsification efficiency of conventional demulsifier. To resolve this problem, a new kind of tannic acid phenol-amine (TAPA) polyether with multiple branches, and aromatic rings is prepared in this article. This synthetic route is expected to avoid the side reactions during the synthesis of commercial phenol-amine resin. Benefit from this structure, the demulsification efficiency reaches 91.7% within 90 min in laboratory test and 97.9% within 45 min on offshore platform measurement toward WACO emulsions. Demulsification mechanism is systematically investigated by the measurements of hydrophile-lipophile balance (HLB) value, relative solubility number (RSN) value, dosage, interfacial activity and stability analysis. To have a direct insight into demulsification, the variation of water droplets in demulsification is observed. The effect of concentration and organic salts on cloud point are discussed. The demulsification effectiveness of TAPA demulsifier is mainly ascribed to the high interfacial activity, hyperbranched structure, multiple aromatic rings and suitable HLB value. These results indicate that TAPA could be an effective demulsifier in WACO emulsions breaking.