In recent years, the breakdown accidents of high voltage cable caused by the ablation of buffer layer in high voltage cable occurs frequently, which has seriously affected the safety and stability of power grid. In this paper, by studying the development process of ablation of buffer layer for 110kV cable, the ultrasonic, electrical and thermal characteristic signals generated by ablation defects are studied, and the evolution process of characteristic parameters of ablation defects is obtained. According to the detection results, the buffer layer ablation is divided into initial stage with only temperature change and rapid development stage with the generation of significant ultrasonic and electrical signals as well as a rapid increase of temperature.