Vegetable insulating oil has gradually become a study hot spot in the replace of traditional mineral insulating oil due to its such advantages as better insulation performance and environmental protection. The terahertz band spectrum has a unique fingerprint characteristic absorption peak

making it one of the hottest technologies in substance detection in recent years. In this paper

the fast

accurate and stable data acquisition characteristics of the terahertz time domain spectrum of insulating oil is used to diagnose the aging state of plant insulating oil. First

the improved electromagnetic wave transfer function is adopted to eliminate the Fabry-Perot（F-P）effect of instruments and vessels. Then

the THz spectrum parameters such as the absorption coefficient and refractive index of the plant insulating oil are calculated and the dielectric properties of the plant insulating oil in the terahertz region are analyzed. Finally

it is compared with the furfural concentration in the corresponding aging state. The results show that the furfural concentration has the same variation trend as that of average refractive index. The research results provide a good reference for the application of THz-TDS technology in the field of plant insulating oil aging assessment.