The present invention discloses an in situ optical settlement sensor comprises three hydrogen-doped Fiber Bragg Grating (FBG) optical fiber, six fiber holders, three 3D mechanical arms and a dome-shape protection case. Besides, the present invention also discloses a method to fabricate the in situ optical settlement sensor. The in situ optical settlement sensor fabricated according to this method is capable of detecting 3-dimensional micro-displacement of the supporting columns of existing buildings and infrastructures to predict the foundation settlement up to 15 mm with 0.08 mm resolution.