The heterogeneous mixed-grain microstructure is a common defect for the heavy forging of 316LN austenitic stainless steel.Isothermal compression experiments were performed on a Gleeble-3500 thermo-mechanical simulator to investigate the effect of process parameters on the fragment and re-finement of millimeter-grade coarse grains (MCGs) during hot cogging.The experimental results in-dicate that the stress of MCG specimens is much larger than that of fine grain (FG) ones at 1150℃, while the stress difference between MCG and FG samples became smaller at 1200℃.Moreover, the MCGs can be well fragmented and refined under the condition of temperature of 1200℃, strain rate of 0.01 s-1 , and reduction rate of 50%.Meanwhile, numerical simulations were conducted to study the influences of temperature, strain and strain rate on microstructure evolution.The results of ex-periments and simulations comprehensively demonstrate that the MCG results in the increase of de-formation resistance and incompatibility of deformation, and it can be fragmented and refined at 1200℃ so that the plastic deformation energy decreases remarkably with the increase of temperature from 1150 to 1200℃.