Research conclusions :(1) The rule of the ground surface settlement for the loess subway shield across the existing Lianyungang - Lanzhou railway was forecasted by the FLAC3D numerical simulation analysis; when shield construction without control measures，the settlement of roadbed upward the right tunnel axial line is 20. 48 mm， the settlement of roadbed upward the left tunnel axial line is 12. 85 mm，they are both beyond the allowable value; when shield construction with control measures for reducing the construction risk of the subway shield across the existing line，such as control earth pressure strictly，shield impelling in uniform speed，control the injected amount strictly，reduce the change of the shield impelling direction， etc. The settlement of roadbed upward the right tunnel axial line is 5. 44 mm，the settlement of roadbed upward the left tunnel axial line is 4. 95 mm，they are within the allowable value. (2) The result of the site monitoring indicates that the deformation rule forecasted by the FLAC accounting match with the true value basically，the deformation of the surface and the railway roadbed is allowed. The measures for reducing the construction risk of the subway shield across the existing line proposed is reasonable and effective. (3) The research results can be used in construction control of deformation for the loess subway shield across the railway.