The sandbag cofferdam at drydock entrance of Zhoutouzui Immersed Tunnel in hard soil foundation in Guangzhou is numerically simulated by 3D finite element model, so as to learn the horizontal deformation rules of sandbag cofferdam under dewatering condition in drydock. And then site monitoring data are collected and analyzed. Meanwhile, comparison between the layout scheme of sandbag cofferdam with multiple folded corners U shape and that of two rightangle corners U shape is made. The results show that: 1) The horizontal deformation rules of different points along axial line of sandbag cofferdam are different under dewatering condition. The view of regarding cofferdam deformation as plan strain is defective. 2) The rigidity different of the cofferdam system induced by plan layout will cause spatial effect. 3) The cracks on cofferdam top are induced by differential deformation which is caused by different rigidities of corners and middle part of cofferdam. 4) The layout scheme of sandbag cofferdam with multiple folded corners U shape is superior to that of two rightangle corners U shape.