Abstract: In order to study the three-dimensional structure of recirculation zones of swirling flow in confined space, realizable k-ε model was used to simulate the swirling flow field in different confined space with the swirl number of 0.884. The confinement ratio is an important factor affecting the shape of recirculation zone. When the confinement ratio goes above 6, the central recirculation zone and the downstream recirculation zone are two independent zones with two pairs of vortices. When the confinement ratio is between 3~6, the central recirculation zone and the downstream recirculation zone are merged together, and there are two pairs of independent vortices. When the confinement ratio is less than 3, a bubble-shaped central recirculation zone is formed in the flow field with a unique pair of vortices. When the confinement ratio is around 3, there will be a transition state, in which the formation process of the recirculation zone is obviously different from other conditions. In this state, vortex core structures of multi helix, single helix and double helix appear successively. The spiral direction of single and double helix vortex procession is opposite to that of the multi helix.