Abstract This paper presents a numerical method for gas–solid two-phase free turbulent flow. The computation of the gas flow by a vortex method and the Lagrangian calculation of the particle motion are simultaneously performed, in which the change in the vorticity for the gas-phase by the interaction between the two phases is taken into account. The change due to the force exerted by the particle on the gas-phase is evaluated by an area weighting method, while the change by the viscous effect is simulated through a core spreading method. The present numerical method is also applied to calculate a two-dimensional gas–solid two-phase mixing layer to confirm the applicability. The numerical results, such as the number density, velocity and turbulent intensity of the particle, are favorably compared with the experimental data.