Abstract: A thermal insulation water layer is designed for the built-in pressurizer of ACP100+ small model reactor. The flow and heat transfer characteristics of heat insulation water layer are researched using numerical method, and the temperature and velocity distributing during power operation steady condition and during the drop power operation transient condition are analyzed. The results show that, the inner fluid is with weak flow and thermal conductive ability, and the structure of thermal insulation water layer isolate the high temperature from the low temperature fluid effectively.