Nanocrystalline magnetic metallic microwave absorbents have been widely used to produce microwave absorbing coatings of thin thickness and low density, since they can simultaneously cause magnetic and dielectric loss to microwave. However, when nanocrystalline magnetic metallic absorbents work at elevated temperature, they generally suffer from three challenges, i.e., increase in amplitude for atomic dynamic vibration, oxidation and grain growth. This work aims to provide systematic understanding to challenges and strategies for magnetic absorbent working at high temperature, focusing on the origination, mechanism and effect of Curie temperature, high temperature oxidation and grain growth. The strategies to suppress disordering magnetic moments, oxidation and grain coarsening are discussed from the perspectives of alloying art and nanostructure controlling; surface coating, formation of dense metal oxide layer to suppress oxygen diffusion and internal structure construction; thermodynamics, kinetics and thermodynamic-kinetic synergy, respectively. This work is constructive to promote high-temperature application of magnetic metallic absorbents.