In　recent　years,　the　decay-like　fracture　of　composite　insulator　core　rods　is　encountered　frequently,　which　has　seriously　affected　the　reliability　of　transmission　lines.　However,　there　is　a　lack　of　assessment　method　to　quantify　the　degrees　of　composite　insulator　core　rod　degradation.　In　this　article,　a　method　for　assessing　the　degree　of　composite　insulator＇s　degradation　based　on　ultrasonic　guided　waves　was　proposed.　The　study　focused　on　core　rods　with　different　decay-like　fracture　degrees　in　composite　insulators　obtained　from　the　field.　Simulations　and　tests　of　ultrasonic　guided　waves　were　conducted　to　assess　the　decay-like　degrees,　and　the　physicochemical　properties　of　composite　insulators　at　different　stages　of　decay-like　development　were　tested　to　verify　the　assessment　results　from　a　microscopic　perspective.　The　results　show　that　the　35　kHz　T(0,　1)　and　55　kHz　L(0,　1)　modes　are　suitable　for　detection,　and　the　peak-to-peak　values　of　the　ultrasonic　guided　wave　packet　received　at　a　certain　distance　decrease　with　the　increase　of　the　decay-like　degree.　With　the　progression　of　decay-like　aging,　the　dielectric　constant,　the　dielectric　loss　tangent,　and　the　carbon　content　in　the　core　rod　increase　significantly.　On　the　other　hand,　the　oxygen　and　silicon　contents　show　the　opposite　trend.　Infrared　spectral　results　can　effectively　distinguish　new　factory　samples　and　samples　with　decay-like　fracture　deterioration.　However,　it　shows　limited　differentiation　capability　for　composite　insulators　at　various　stages　of　decay-like　aging.　The　research　provided　a　reference　for　assessing　the　decay-like　degree　of　composite　insulator　core　rods.