In order to obtain an effective online monitoring method for T-shaped cable terminal joints deterioration, this paperfirstly simulates and analyzesthe electric field distributioncharacteristics ofT-typecablejointsunder typicalairgap moisture defects, and proposes a defect monitoring and identification method based on changes in electric field signals. Then the electric field signal monitoring test of the lower T-type cable terminal joint is carried out to verify the proposed method. Research results show that, for the deteriorated T-type cable terminal joint, when the humidity is sufficient, the effective value of the space electric field increases by 200%; and when the internal moisture causes partial discharge, the effectivevalueofthespaceelectricfieldincreasesby5~6times.Bycomparingtherisingrateoftheelectricfieldintensityin themonitoringlocation, itcanidentifywhethertheT-shapedcablejointhassignsofdeterioration.