Abstract: The local low hardness P91 elbow found in the pipeline survey of a power plant was taken as research object.On the one hand, through grid-based hardness testing and data acquisition, the low hardness area of the elbow is divided by image processing method, and then the maximum stress in the low hardness area is obtained by finite element calculation.On the other hand, according to the on-site hardness and metallographic test results, by referring to relevant literature, the lasting strength of the same grade hardness is obtained, and finally the remaining life of the elbow is calculated by isotherm extrapolation method. For local low hardness P91, the creep life assessment method based on image processing method can achieve accurate Ⅱgrade life assessment, which can provide a way of thinking for the general power plants to deal with local low hardness P91 problems, which also has certain guiding significance for the life management of local low hardness P91 in service units.