Abstract: The structural thermal expansion of the heat pipe cooled reactor will significantly affect the heat transfer and neutron physical transport process. A geometric updating strategy and a reactivity feedback method considering the significant expansion of the solid core is presented in this paper, and a neutron physics/thermal/mechanical coupling algorithm based on dynamic geometry is established. In the coupling, the change of the cross section caused by temperature, the change of the density of the materials and the change of the core size caused by thermal expansion are considered. Furthermore, the coupling analyses are applied to the MegaPower heat pipe cooled reactor. The convergence of the core power distribution and the radial power factor under different relaxation factors is analyzed. Results show that the neutron leakage of the side-channel increases due to thermal expansion, which generates negative reactivity feedback and yet aggravates the non-uniformity of the power distribution and the deterioration of the heat transfer. The radial power factor increases from 1.20 to 1.23 when considering the thermal-mechanical feedback. The peak fuel temperature increases by 11 K.