The invention relates to a method for treating ageing oil of viscous oil, which provides a demulsifier for ageing oil of viscous oil, comprising the following raw materials in weight percent: 0.5-1% phenol, 1.5-2% styrene, 0.5-1% ethyleneamine, 0.5-1% formaldehyde, 0.5-1% catalyst, 40-50% ethylene oxide and 40-50% epoxypropane. The invention further provides a three-phase decanter centrifuger, comprising a differential mechanism, a spiral propeller, a rotary drum, a housing, an engine base, a motor and a feed pipe. The invention further provides a method for treating ageing oil of viscous oil, comprising the following steps of: using the demulsifier to perform demulsification treatment to the ageing oil of the viscous oil, then inputting the ageing oil of the viscous oil after the demulsification treatment in the three-phase decanter centrifuger to centrifuging and separating so as to respectively obtain fatlute, sewage and the oil. The ageing oil, especially the ageing oil of the viscous oil, is treated by adopting the demulsifier to combine with the centrifuging method; the demulsification and dehydration problem of the ageing oil of viscous oil in the oil field is solved, and the requirement of the field production can be satisfied.