Swirling gas-particle flows in a co-axial sudden-expansion chamber with different swirl numbers are simulated using an improved second-order-moment two-phase turbulence model. The particle Reynolds stress equations and the two-phase fluctuation velocity correlation terms are closed based on a Lagrangian analysis, accounting for the crossing-trajectory effect, inertial effect and continuity effect. Predictions give the gas and particle axial and tangential averaged and fluctuation velocities for swirl numbers of s = 0, s = 0.47 and s = 0.94. The swirl number is defined as the ratio of tangential momentum to axial momentum. Prediction results are in good agreement with the PDPA measurement results for both mean and fluctuation velocities of single-phase swirling flows and two-phase mean velocities of swirling gas-particle flows. However, the normal components of Reynolds stresses or the fluctuation velocities of two phases for swirling gas-particle flows are still underpredicted. The results show that increasing swirl number changes the shape and sizes of recirculation zones, the size of the solid-body rotation zone, reduces the turbulent fluctuation of two phases in the upstream region and enhances it in the downstream region.