The Laser Doppler Velocimetry (LDV) or Adaptive Phase Doppler Velocimetry (APV) system was used to study the water mist flow field under longitudinal ventilation. The water mist characteristics: mean velocity, volume mean size and volume flux were obtained. The results indicate that in the same cross section mean velocity of water mist increases as airflow velocity increases, but displays as two step linear fitting. Experiments deepened the understanding of the mechanics of water mist induced by a jet and provided better conditions for experiment of water mist induced by flame in the next step.