The bulk copolymerizaition is an effective method to produce ethylene-vinyl acetate (EVA) copolymer. In this paper, the thermal risk of the EVA copolymerization was studied using vent size package 2 (VSP2) in order to avoid thermal runaway of the reaction. The effect of pressure, residence time, initiator concentration and heat input on the runaway reaction under adiabatic condition was investigated. The emergency relief system of the EVA reactor was sized. The results show that the EVA copolymerization reaction is autocatalytic and greatly influenced by pressure. The increase of the amount of initiator will significantly increase the reaction rate, thus is the worst case for relief system sizing. This study will provide the safety basis for the industrialization of the EVA bulk copolymerization process.