The ooidal ironstones (OIS) are distinctive non- cherty, sandy clayey siliclastic or siliclastic- carbonate sedimentary rock with more than 50% of ferruginous ooids and more than 15% iron. Middle- Late Devonian OIS, named as the "Ningxiang type" iron deposit are widely distributed in the Upper Yangtze area, South China. It is, therefore, an excellent research object to reveal the formation mechanism of OIS. Based on field and microscope observations, this contribution conducts detailed morphology and geochemistry analyses by SEM and EPMA on OIS samples from the Huoshaoping iron deposit in the western Hubei Province, Central China. Our results prove that Ferruginous ooids from the deposit were generally developed in transition zone from shoreface to offshore, produced in silty- sandy carbonate facies, and are most abundant in the distal offshore transition zone and the proximal offshore zone. Three main industrial iron layers of the deposit are strongly developed in fast- transitional zone of transgression- regression- transgression in seawater transgression cycle. Combined with the sedimentary environment of ooids, we further support the colloidal chemical genesis model of ooids, and derive a model for OIS based on the Huoshaoping iron deposit: ① During transgression, the eutrophication of seawater caused by enhanced weathering made the deep seawater present anoxic reduction environment, and Fe2+ could be enriched. ② The short- term regression not only provided the source of core materials for the formation of ooids, but also changed the dynamic and redox environment of seawater, which led to the deposition and growth of ooids. ③ Another transgression resulted in the deposition and burial of OIS.