Abstract: Taking a new developed cold work die steel SDC99 C-ring specimen as the research object and combining with the low temperature physical parameters and the boiling heat transfer coefficient acquired by an empirical formula method, a finite element model based on metallo-thermo-mechanical coupled theory was built up to simulate the deep cryogenic treatment(DCT)for a C-ring specimen and to explore the essential characteristic of temperature and microstructure evolution. Results show that during DCT, the differences in temperature and cooling rate between the gap and core regions of C-ring specimen were very significant, especially in the cooling rate. Subjected to DCT, the amount of retained austenite contained in C-ring specimen markedly decreased from 15 vol.% at initial to 2.3 vol.% at last, while the volume fraction of martensite increased from 85% to 97.7%. After DCT, the retained austenite nonuniformly distributed but principally congregated near the gap region of specimen. Compared with the quantitative phase analysis by XRD, the predicted results present a quite good accuracy.