Fire accidents are considered serious events for tunnel safety since they can cause heavy losses as well as serious damage to the tunnel infrastructure and facilities. Although risk assessment prepares the tunnel system to deal with potential fire accidents, an adequate response by tunnel operators depends also on the information about a particular fire event. This paper proposes a novel risk-based method that supports tunnel operators in assessing the criticality of potential fire events. The structure of the proposed method is as follows. Initially, the factors that determine the criticality of a fire event are featured and the system parameters that affect these factors are identified. Subsequently, the method performs multiple simulations by changing the examined parameters randomly and thus the relation between the factors and the parameters arises. The outcome facilitates tunnel operators to predict promptly the severity of potential fire events and make better-informed decisions.