The 18.5% Cr low nickel type duplex stainless steel with high manganese content was compressed by using thermal simulation test machine with large deformation of 70% under deformation conditions of 1123~1423 K/0.01~0.1 s^-1, while the microbstructure characteristics and softening mechanism of two phases during thermal deformation were investigated. The results show that the thermal compression softening in the range of 0.01~0.1 s^-1/1123~1223 K was dominated by recrystallization of ferrite phase, while in the range of 0.1 s^-1/1323~1423 K and 10 s^-1/1223 K was dominated by recrystallization of austinite phase. When deformed at 1223 K and 0.01~10 s^-1, the dislocation tangles in the ferrite phase evolved into dislocation cells and the dislocation lines appeared with the increase of strain rate, and the substructure of austenite phase transformed into fine recrystallized grains. When deformed at 0.1 s^-1 and 1123~1323 K the substructure of the dislocation cells gradually formed due to the increase of dislocation density in ferrite phase with increasing deformation temperature, but the deformation microstructure in austenite phase changed from DRV to DRX with the decrease of dislocation density. The deformation apparent activation energy Q and the apparent stress exponent n were calculated as 514.29 kJ/mol and 7.13 respectively based on thermal deformation equation, and the constitutive equation with Z parameter was established. Meanwhile, the critical conditions of DRX have been obtained by the relationship between work hardening rate and flow stress, and the relationships between Z parameter and the critical conditions were also determined. The hot working map analysis shows that the instability zone gradually decreases with increasing deformation strain, and the optimal processing zones are within the range of 1348~1432 K/1~10 s^-1, and corresponding values of power dissipation coefficient are above 0.4.