Swirl cups with three different flowing area ratios were designed. The spray velocity fields of the swirl cups were firstly studied by particle imagine velocity (PIV) under selfstimulated condition of air pressure. The effects of the flowing area ratio on the spray velocity field and the distributions of velocity were studied and analyzed

respectively. The results shows that the area and length of central recirculation zone gradually become smaller and shorter

and the recirculated velocity of central recirculation zone and the angle of spray are tended to be higher and smaller with the increase of the flowing area ratios. In addition

it can be seen that the capacity of antideviation increases with the increase of flowing ratios according to the radial distribution of spray velocity.