Abstract: In order to achieve high-precision and high-fidelity numerical simulations of nuclear energy system and to explore the true physical process therein, a coupled three-dimensional high-fidelity calculation platform named MPCH for neutronics/thermal-conduction/stress-analysis calculation has been developed in this work. Therefore, MPCH can be used to perform a series of multi-physics coupled calculation of neutron transport, thermal conduction and thermal expansion. MPCH is constructed based on Picard iteration by integrating open-source codes: OpenMC, Nektar++ and SfePy. In this paper, the new space heat pipe reactor KRUSTY is calculated and analyzed under MPCH. The calculation results of multi-physics coupling show that MPCH can effectively predict the effective multiplication factor change, power distribution, temperature distribution and thermal expansion of the KRUSTY reactor. At the power of 4 kW, the local temperature difference of the whole reactor is 21.6K. The thermal stress leads to the deformation rate of 2.47%. Moreover, the effect of neutronics/thermal-conduction/stress-analysis coupling tends to make a more uniform temperature distribution of the core. This multi-physical coupling calculation program plays an important role in the design, development and verification of new reactors.