Thermal runaway reactions of organic peroxides often cause accidents. To improve the chemical production safety, pressure vessel test studies were conducted on the thermal runaway reaction of di-tert-butyl peroxide (DTBP). The effects of furnace temperature and sample mass on the runaway reaction characteristics were discussed, and the venting phenomenon was captured by high-speed camera. The results indicated that the thermal runaway reaction of DTBP includes thermal decomposition, explosion, and venting. The combined effects of heat release, gas production, and boiling that occur simultaneously increase the complexity of decomposition process. If DTBP is completely vaporized before venting, an explosion is triggered. For the same sample mass, the higher the furnace temperature, the faster the rise in temperature and pressure, and the shorter the decomposition reaction time. For the same furnace temperature, the larger the sample mass, the later the thermal decomposition, and the greater the pressure rise rate after boiling. Furthermore, as the furnace temperature increases and the sample mass decreases, it is more likely to cause an explosion, whereas there are no significant changes in the venting characteristics during the pressure drop process. This work can provide a reference for process safety and risk control of organic peroxides.