To examine the gas dispersion mechanism around a disk turbine impeller, the cavity structures behind impeller blade were studied from the range of a vortex cavity and a clinging cavity to a large cavity, An approach named the "minimum pressure point connecting method" was developed and it was adopted to depict the locus and conformation of the trailing vortex, while the shape of the large cavity was obtained through the experimental observation. The variations in the calculated pressure, deformation rate and shear stress along the axis of the trailing vortex were illustrated. The bubble sizes and bubble size distributions were measured at various locations around different type cavities to discuss how the gas entering the impeller is dispersed. For the vortex cavity and clinging cavity, the largest values of the forces related to gas dispersion, such as deformation rate and shear stress, always appear at the cavity tail, which indicates that most of the total gas passing through the impeller was dispersed at the tail of the cavity, From the results obtained from bubble size measurements, it is found that for the vortex or clinging cavities, approximately 60% of the total gas is dispersed at the cavity tail, and about 15% of the total gas is dispersed at the circumference of the cavity, However, for a large cavity, more than 80% of the total gas is dispersed at the cavity tail and almost no gas is torn at the edge of a targe cavity, which indicates the diminution of tile intense rotary motion of a targe cavity.