Abstract: In this article ,weak soil is discussed on specially designated environment of sedimentation and particular material form ,it is indicated that weak soil have characters of physics and mechanics such as high natural water content,big void ratio,low shear strength,high compressibility coefficient 、low osmotic coefficient 、low foundation bearing capacity 、big deformation and non-homogeneous deformation ，besides ,consolidation settlement stabilize time is longer .In seaside district , consolidation settlement and precipitation influence are direct reasons giving rise to earth subsidence ,human activity is main predisposition of earth subsidence .Analysis earth subsidence of building and structures in two typical estates by observation result ,it is illustrated that secondary consolidation settlement of weak soil under building and structures in activate district on marine reclamation land is not to ignored .Rationalization proposals in deal with earth subsidence on different function land are put forward in the paper .In construction and lifetime ,setting up a perfect observation system and making settlement monitoring are most important engineering measure.