The objective of the invention is to provide a residual life assessment method of a transformer. The method considers the qualitative information which influences the life of the transformer, establishes the residual life characteristic parameter index system of the transformer, constructs the membership functions of each index, determines the weighting of each index parameter, combines the grade of membership and the weighting of each index, and calculates the health state value at the current state so as to calculate the residual life of the device to be assessed.