Abstract: A physical description of the formative cause and mechanism of the droplets movement in the steam uniform flow field in the separator was researched in the paper. According to the mechanism of gravity separation, the forces on droplets were analyzed and the equation of single droplet was founded and solved. The effects of the velocity of the vapor flow，the droplet diameter, the initial velocity as well as the gravity separation space height，on the gravity separation efficiency were discussed in details. The mechanism of the droplets movement and separation qualitative analysis will set up the groundwork for the qualitative analysis of the dynamic single-drop model.