Hot ring rolling (HRR) is a 3D unsteady-state and coupled thermo-mechanical process, the metal undergoes complicated unequal deformation and microstructure evolution. In this paper a 3D rigid-plastic and coupled thermo-mechanical FEM model for hot ring rolling was developed based on DEFORM3D platform, taking dynamic recrystallization (DRX) volume fraction, DRX grain size, recystallization volume fraction and average grain size as objects, the mechanism of material microstructure evolution and distributions in HRR process are thoroughly studied. The results show that: with the HRR progressing, the DRX volume fraction, volume fraction, DRX grain size and average grain size have the similar distributing characteristic, and the distribution zones expand from a small local area into the whole ring strip, then diffuse to the mid-layer of ring with small deformation, their distributions become more uniform. Meanwhile with increase of deformation, the values of the DRX volume fraction and recrystallization volume fraction augment, i.e. the degree of recystallization increases. The DRX grain size also augments due to local high temperature, while the average grain size decreases. In general during HRR process the distributions of DRX volume fraction, recrystallization volume fraction, DRX grain size, and average grain size are ununiform due to unequal deforming in HRR process.