Siliciclastic grains, characterized by concentrated Al, Si, K and other elements, are typical components of ooids, but their distribution and contribution to ooid formation are uncertain. Coatings have played a significant role in small shelly fossil preservation in early Cambrian phosphate or phosphatic carbonates. However, the relationship between ooid construction and the preservation of small shelly fossils has not been fully elucidated. Herein, we report well-developed flaggy ooids containing siliciclastic grains from a middle Cambrian carbonate–siliciclastic depositional system in North China. We observe that ooids consist of multiple layers of dark and light laminae. The dark laminae with a high organic content and flexible appearance are interpreted as biofilms. The distribution of detrital grains is closely coupled with these dark laminae and the filamentous structures around fossil shells, which indicates that the detrital particles are bound by the biofilms or filamentous structures. The high occurrence and compacted nature of siliciclastic particles observed in these ooids reveal that detrital grains could be the main contributing factor in ooid formation in the middle Cambrian Longwanggou section. Those flaggy ooids composed of clay-size detrital grains finely mimic the shell morphology and microstructure, providing another important window for the preservation of small shelly fossils in the Cambrian.