The invention discloses a segment assembling system for a rectangular shield. The segment assembling system comprises two assembling machines, wherein a rectangular tunnel comprises two assembling space parts, and the two assembling machines are arranged in the two assembling space parts respectively; each assembling machine comprises an axial translation mechanism, a swing mechanism, a lifting mechanism, a horizontal translation mechanism and a fine adjusting mechanism, the axial translation mechanism comprises two axial guide rails arranged in an axis direction of the shield as well as fixed disks which are arranged on the two axial guide rails and can move along the axial guide rails, the swing mechanism comprises rotary disks which are arranged on the fixed disks and can rotate around the fixed disks in the axis direction of the shield, the lifting mechanism comprises two guide seats symmetrically arranged on the rotary disks as well as lifting columns which are arranged in the guide seats and can move in axis directions of the guide seats, the horizontal translation mechanism comprises racks fixed on the two lifting columns in the direction perpendicular to the axis directions of the guide seats, two horizontal guide rails arranged in the racks and lifting frames which are arranged on the two horizontal guide rails and can move along the horizontal guide rails, the fine adjusting mechanism comprises spherical hinge oil cylinders retractably arranged at bottoms of the lifting frames as well as a plurality of deviation adjusting oil cylinders, lifting appliances are arranged on the spherical hinge oil cylinders, and cushion blocks are arranged on the deviation adjusting oil cylinders.