Abstract: The AP1000 uses gray rods whose neutron absorption capability is lower than that of black rods to achieve load follow without requiring changes of the boron concentration. The use of gray rods results in reducing the amount of wastewater. Taking gray rod assembly in the AP1000 reactor as calculation model, transport-burnups were calculated by using the TRITON code of the SCALE5.1 code system. The results show that in the gray rod assemblies with different enrichments, there is large difference in the eigenvalues at the beginning of fuel cycle between gray rod assemblies composed of different absorption materials. With the deepening of burnup, the difference between them becomes smaller and the relative deviation is less than 1%, the neutron absorption cross section of tungsten rods changes gentler relatively. The results provide the reference for the actual operation of AP1000 reactor.