A series of tests was carried out to investigate the influence of different parameters on design fires for tunnels with water-based fire suppression systems in a 1:4 scale tunnel. The key parameters tested included fuel load covers, activation time, water density, nozzle type, ventilation velocity, sprinkler section length and tunnel width. Analyses of test data are presented with a focus on the influence of these parameters on the design fire. The main findings are that the fuel load covers, activation time and ventilation velocity all play significant roles in the fire development. Further, suggestions on design fires for tunnels with water-based fire suppression systems are proposed. © 2020 The Authors