Along its midline are scales or scutes with a linear ornament, alongside two dorsal fins of similar size, both bearing spines.

The skull roof's lateral lines are gently curving like those of "maxillate placoderms" and acanthothoracids.

The head shield resembles placoderms such as Lunaspis and Romundina closest in general shape.

[1] Xiushanosteus derives from Xiushan County where the fossils were found, alongside osteus "bone", a common suffix for placoderm names.

The species name mirabilis translates to "miracle" due to how unexpected the find was.