The core physics properties of 30 kW In-Hospital Neutron Irradiator-Mark 1(IHNI-1) reactor loaded low-enrichment fuel are simulated by using MCNP/4B Monte Carlo code. The arrangement scheme of core, ²³⁵U enrichment, control rods' value, excess reactivity and shutdown margin are also reasonably introduced. The results show that the physics scheme of IHNI-1 reactor which possesses some particular characteristics, such as high inherent safety, 10 years lifetime without any refueling and low-enrichment UO₂ fuel, has been discovered in the paper and it will provide theoretic basis for reactor engineering design and neutron beam design for Boron Neutron Capture Therapy in the future.