A series of tests were carried out to investigate design fires for tunnels with water-based fire suppression systems under different conditions in a 1:4 model scale tunnel. The key parameters including fuel load covers, activation time, water flow rate, nozzle type, ventilation velocity, sprinkler section length and tunnel width were tested. Technical information and analyses of test data are presented with a focus on the influence of these different parameters on the design fire in a tunnel with a water-based fire suppression system. Further, guidance for the design fires in tunnels with water-based fire suppression systems is proposed.