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The effect of refined precipitations and dispersed phases on the toughness of SS400 steel was investigated by rapid tempering with thermomechanical simulation tester, and the electromagnetic induction rapid tempering process was simulated. The conventional tempering and rapid tempering process were proceeded respectively, and both samples were quenched in 10% of agitated iced brine. The tempering temperatures were designed as 560, 620 and 680 ℃, respectively. Rapid tempering specimens were heated at a heating rate of 20 ℃/s, and all samples of these three tempering temperatures were maintained 30, 40 and 50 s with the Gleeble1500-D tester, respectively. The impact test at -40 ℃ were carried out on a Charpy impact machine (CBD-300) with a maximum measurement range of 300 J and the microstructures were analyzed in detail using optical microscope (OM) and scanning electron microscope (SEM). The experiment results show that the upper bainite, martensite and small amount of austenite were obtained in the rapidly quenched samples. In comparison to the conventional process, the matrix microstructure was changed from the larger size and bulk ferrite to the refined banding structure, and the cementites were obviously refined and precipitated inside the grains and along the grain boundaries. The cryogenic impact test implies that the maximum values of impact toughness with holding time of 30 or 50 s are achieved at tempering temperature of 620 ℃. Furthermore, a favorite value of impact toughness with holding time of 40 s is available.