Abstract: Nuclear power plant CRDM (Control rod drive mechanism) thermal sleeves represent one of the critical components of reactor pressure vessel heads, whose stability and reliability directly affect control rod operation states, making their quality inspection a key and challenging aspect of in-service nuclear plant inspections. CRDM thermal sleeves feature variable-diameter special structures, posing significant difficulties for ultrasonic testing. Dedicated phase coherence imaging (PCI) technology was studied for CRDM thermal sleeves, utilizing ultrasonic signal phase information for high-resolution imaging and detection, with comparisons made between total focusing imaging and phase coherence imaging to analyze imaging differences on variable-diameter structures and detection effectiveness for typical planar defects. Results indicated that phase coherence imaging provided better imaging for defects near variable-diameter structures, with more obvious endpoint diffraction signals for planar defects, facilitating defect qualification and precise quantification.