This study utilized ground‐based Doppler radar observations to document the detailed structures of an outer tropical cyclone rain band (TCR) of Typhoon Longwang (2005) as the rain band passed over the topography of northern Taiwan on 1 October 2005. The TCR was oriented northeast–southwest and was roughly parallel to the mountain barrier with a higher (lower) terrain height in its southern (northern) landfalling segment. During the pre‐landfall stage, the TCR was characterized by convective precipitation on its inner side, where the front‐to‐rear flow met with the rear‐to‐front flow to lift air upward with overturning‐updraught‐like signatures. Significant (relatively minor) changes in the TCR's structures were evident in the southern (northern) landfalling segment during the landfalling stage. In particular, the southern landfalling segment exhibited a much stronger front‐to‐rear flow that passed through the region of the rain band, resulting in the absence of low‐level organized convergence and a relatively disorganized nature of precipitation. This structural change was shown to be closely related to a significant decrease in the cross‐barrier flow due to the orographic blocking that occurred as easterly ambient winds encountered local high mountains in the vicinity of the rain‐band segment. As the TCR propagated over the western side of the major topography, the low‐level rear‐to‐front flow reappeared due to the collapse of low‐level orographic blocking, and the convergence between the front‐to‐rear flow and the rear‐to‐front flow tended to rebuild a well‐organized convective circulation along the rain band. Results from this case‐study provide insight into processes that contribute to rapid structural changes in a landfalling TCR over mountainous regions.