The sixth line of Guangzhou Metro Subway is partially located in silty fine sand layer, which is incompact, rich in water, and has high permeability and low bearing capability. When tunnelling in this section, it is easy to bring about gush of water and sand, which will cause the ground surface to settle. Therefore, a two-staged helical conveyer was used to control this phenomenon during tunnelling. First, both the dregs outlets of helical conveyer are turned off to prevent water and sand from rushing forth, then foam material is injected into the earth cabin and the front-end of helical conveyer to keep the cabin pressure from decreasing. Secondly, drain outlet of the second helical conveyer is turned on and the water is drained, then dregs outlet of the second helical conveyer is turned on to discharge the dregs. Finally, dregs outlet of the first helical conveyer is turned on to discharge the dregs in the earth cabin. Such technique was used successfully in tunnelling the section from Datansha to Ruyifang of the sixth line of Guangzhou Metro Subway. It ensured that sinking of the ground surface was avoided while the tunnel shield went through the silty fine sand layer that is rich in water.