As one of the significant geological units in the Helan Mountain area, the Suyukou Formation has long been considered to be of the coastal beach facies. In this study, lithology, facies, biota and other new findings show that it was deposited in the alluvial fan facies. An erosion surface is located in the base of the alluvial fan, which is characterized by complex gravel, heterogranular texture, graded bedding, and turbidity current. Zircon dating of diabase yielded ages of 530～540 Ma, suggesting an early cracking basin, which probably resulted in the formation of the alluvial fan in the Suyukou Formation.