Abstract: To investigate the effects of venturi tube structure on flow field characteristics of the combustion chamber, three-dimensional numerical simulation was carried out to a single stage axial swirler/venturi tube. Results show that as the divergence angle increases, both the radial expansion intensity and diameter of recirculation zone increase; when the venturi tube's divergence angle and contraction ratio change, double-, triple- and quadruple-vortex will occur in the recirculation zone, and the vortexes distributed in tandem are equivalent to the main and secondary vortex, bringing benefits to the ignition and combustion of the combustion chamber; the length and back-flow velocity of central recirculation zone increase with the rise of venturi tube's contraction ratio; the structure of triple- and quadruple-vortex is more stable than that of double-vortex, which helps to extend the range of available load, and therefore improve the performance of the combustion chamber.