Abstract: Based on the design structure and manufacture procedure of the coupling structure of CRDM（Control Rod Drive Mechanism） nozzle to RPV（reactor pressure vessel） closure, which was widely used nowadays, this paper focused on the detailed analysis of nozzle uprightness change after RPV hydraulic pressure and the replaceabiltiy of damaged nozzle. Based on the main reasons of nozzle uprightness change and key factor to lower the replaceabiltiy of nozzle, an improved design of coupling structure was produced. It was shown that the improved coupling structure could ensure the nozzle uprightness and lower the difficulty to replace the nozzles.