An in-line planar fuel cell height measurement system is mainly composed of a main stand, at least a top platform above a high temperature furnace, a displacement detection unit on the top platform with a central axis connecting to an extension rod that goes downward into the high temperature furnace and contacts the top surface of the cell stack inside the high temperature furnace, a displacement display unit connecting to the displacement detection unit through signal transmission cables, and a data processing unit connecting to the displacement display unit through signal transmission cables, so the displacement detection unit can sense the height change for the cell stack in the high temperature furnace during temperature rise and operation and send out a signal, which can be directly displayed by the displacement display unit and received by a data processing unit for further analysis and storage.