An onsite fire simulation experiment is conducted in Qiujiayakou Tunnel on Zhaohui Highway in Yunnan province to verify the performance of low pressure water mist fire extinguishing system to suppress fire spreading and expending during early fire in tunnel. During the experiment, the temperature and gas fields after low water mist spraying are measured; and fire protection and control performance of low pressure water mist in tunnel fire is studied. The quantitative test data obtained from the fire simulation experiment show that fire flame has been controlled, the temperature field around fire source has been reduced, the concentration of CO reduced rapidly after reaches peak value, and concentration of O2 kept approximately 21% during experiment, which can guarantee the fire safety and evacuation of personnel.