Smurf attack belongs to popular Denial-of-Service (DoS) attack, and they can cause devastating impact on computer systems. Inspired by the principles of immune danger theory, a novel risk evaluation model, referred to as DTRESA, for smurf attacks is proposed in this paper. Within the presented model, dangerous smurf attacks are compared to bacterium (or virus) of the immune danger theory, which induce danger signal by simulating cellular distress or cell unnatural death; through immune recognition of artificial lymphocytes, the attacks are detected, and the attack risk is evaluated by calculating the danger signal of host computers. Simulation results and theoretical analysis show that the proposed model is feasible. Thus, it provides a novel solution to DoS detection and computer network security risk assessment.