Abstract: For structures and assembly forms of the nuclear reactor core used in marine nuclear power platform, COBRA which is a proved sub-channel analysis code was used to verify the thermal-hydraulic security characteristics of the reactor core. When the reactor works at the pressure of 14.8 MPa, the calculated results of the steady state show that the minimum value of departure from nucleate boiling ratio (DNBR) is 2.342, the highest surface temperature of fuel rod cladding is about 342 ℃, and the highest temperature of the center for pellet is about 1545℃. It is shown that thermal-hydraulic characteristics of the reactor can meet the requirements of security criterion and leave enough safety allowance for the impact of complicated marine conditions. At the same time, a sub-channel analysis code was developed to check the COBRA code results and it is ensured that the results are enough reliable and accurate. Through comparing the results with each other, the good agreements of the comparison proved in some degree that the results are reliable. According to above calculations, we can draw a conclusion that the steady thermal-hydraulic characteristics of the reactor assembly are safe and the results are reliable.