The difference between general trapped vortex combustion and CIAC （cavity in a cavity）trapped vortex combustion of ITB (Inter-Turbine Burner) 3-D combustor on two phase combustion flow field and combustion performances are studied by using CFD software. Realizable model is applied to simulating the turbulence. Non-premixed combustion model is used to model the turbulence-chemistry interaction. Then, the analysis and comparison of the temperatures, flow field and species are done in the two different ITB structures. It is found that the ITB of the two structures can yield a substantial increase in the entrance temperature and speed of low-pressure turbine by the results. The temperature of the front half section is higher than that of the rear half section in the circumferential cavity. The fuel participating in the secondary burning in the downstream of the main path way is an incomplete burnt, incomplete oxidized low carbon compound. A fuller combustion and a greater temperature rise are obtained in CIAC (cavity in a cavity) structure. The groove on the top has a well effect on stability of combustion.