Abstract: The thermoplastic forming limit of high strength boron steel, 22MnB5 at high temperature was studied by both experimental research and numerical simulation. Firstly, the device, TFLD300, was developed for the forming limit test based on the Nakajima test. A series of forming limit tests at high temperature were carried out and forming limit curves at different temperatures were obtained. Then a 3-D thermal forming limit diagram with temperature history and strain path was constructed accordingly. Based on the independently developed metal forming commercial software KMAS (king mesh analysis system), the thermoforming analysis module used to deal with the coupling relationship among thermoforming, force, and phase-change was developed with the 3-D thermal forming limit diagram as the criterion for the formability at high temperature. Finally, the typical B-pillar's hot forming process was simulated and the simulated results were compared with the corresponding experiment results. The comparison show that the KMAS thermoforming numerical analysis results are in an accurate agreement with the 3-D TFLD measured results, indicating that both of them can be applied to analyze and measure the fracture of high strength sheet metals in their hot forming.