In the air gap membrane distillation, the rotary tangential flow of outlet was used to scour the membrane surface，and the structure parameter δ and βwere adjusted to increase the turbulence intensity near the membrane surface. As result, the boundary layer thickness near the membrane surface was reduced，the concentration polarization and temperature polarization phenomenon were reduced , and the flux was increased. Fluent software can be used to simulate fluid flow field, temperature field, pressure field and flow state near the membrane surface. The concentration polarization and temperature polarization phenomenon near the membrane surface under differentδ、βcan be deduced by comparing the contrast groups of the flow field. So the optimal structure of the hot-cavity may be obtained.