XLPE cable in practical application is subjected mostly to thermal stress， which would cause an irreversible damaging and shorten its lifetime. A method based on nuclear magnetic resonance (NMR)is proposed. This paper presents the characteristic quantities of thermal aging process-vertical relaxation time and area of wave crest. Experimental results indicate that the vertical relaxation time and area of wave crest decreases with the increase of aging time and aging temperature， and the relationship between the area of wave crest and aging time is approximately linear. The proposed method provides great application value for monitoring cable aging process.