I need to move object by displacement option in Ansys solver but the dimension of bounding box of the object remains constraint for the process, is there any other method to do it ?
Try not to use boundary box, i.e. don't use the "boundary option.
Instead, define a new geometry and model a box. Then use that box as the boundary box. Make sure the new the boundary box has a dimension much much bigger than the object you are considering.