Thermal runaway can occur during the styrene bulk polymerization process because of easily formed local hotspots resulting from the high viscosity of reactants and the difficulty of heat dissipation. To obtain the thermal hazard characteristics, the polymerization behavior of styrene was investigated using differential scanning calorimetry (DSC) at a scanning rate of beta = 2 degrees C/min. Experimental results showed that the exothermic peaks obtained for heat initiation were different from those obtained when initiator was added. The exothermic peak changed from one to two after the initiator was added. The exothermic onset temperature (T-0) was also reduced. Phi-tech II was utilized to study the bulk polymerization of styrene in an adiabatic environment. The adiabatic temperature rise (Delta T-ad), starting temperature of uncontrolled polymerization (T-star), maximum temperature (T-end), and heat of polymerization (Delta H) under different conditions were acquired. When the dose of the additive was increased, the starting temperature of uncontrolled polymerization (T-st(ar)) decreased and the adiabatic temperature rise (Delta T-ad) increased gradually. Severity grading was performed based on the severity evaluation criteria of runaway reaction. The results can help designers decide whether it is necessary to take certain measures to reduce risk.