The invention relates to an assessment method of a voltage sag integration severity based on an energy index. The method is characterized by calculating a system side voltage sag event severity basedon an energy index; establishing an equipment side fault membership function based on the energy index; and establishing a voltage sag integration severity based on the energy index. In the invention,the voltage sag influences of a system side and an equipment side are integrally considered, and through the compatible relation of the two, the voltage sag integration severity is calculated. When the equipment fault membership function is calculated, a fault probability model based on the energy index is used to realize accurate assessment of the voltage sag integration severity, and problems that assessment is not accurate and an assessment process is tedious in an existing index are effectively avoided. The method is of great significance to improving power grid power supply quality, reducing economic losses and increasing a user satisfaction degree and the method is suitable for large scale assessment scenes.