In order to quantitatively evaluate the impact of the overall migration process on the ancient buildings, a method for monitoring the migration of ancient buildings based on ultra-weak fiber Bragg grating (UW-FBG) arrays is proposed. The principle of strain and nonuniform settlement monitoring of UW-FBG sensor array is analyzed. A UW-FBG strain sensing cable with twisted steel cables is designed and its sensing characteristics are calibrated, which center wavelengths are 1530 nm, 1542 nm, and 1552 nm. The fiber optic cable has a resolution spacing of 0.2 m, a strain sensitivity of 1.10 pm/με, and a temperature sensitivity of ~22.4 pm/℃. The fiber optical cable is pre-buried into the pallets of an ancient building in Jing'an District, Shanghai, and an automatic monitoring system based on UW-FBG is constructed to monitor the lifting, migration and placement of buildings in real time. Results indicate that the maximum strain during the relocation of the ancient building is ~110 με, which is less than the pallet strain design threshold of 500 με, and the maximum nonuniform settlement is ~8 mm, of which the lifting process produces the greatest strain impact. The proposed method can realize multi-point, wide-range and high-precision monitoring of the strain field of pallet beams, and obtain the nonuniform settlement of pallet beams in real time, which provides a basis for optimizing the relocation of ancient buildings.