Basin prototype in the Helan—Zhuozi Mountains located in the northwestern margin of Ordos Basin has long been controversial. Therefore, as a link connecting basin and orogen, the characteristics of the Ordovician sedimentary response in the northwestern margin of Ordos Basin should be brought to the forefront. The dual structure of the Ordovician sedimentary assemblages in the northwestern margin of Ordos Basin is ubiquitous——the lithology of the lower part is carbonate rocks of platform facies, while the upper is characterized by a set of flysch deposits, which represents different stages, from extension to aggregation, in the evolutionary process of the basin prototype. Methods: Combined with the Ordovician geographic background and regional tectonic evolution, this paper studies two representative sections in the Zhuozi Mountain and Helan Mountain area, which are located in the northwestern margin of Ordos Basin. The evolution of the Ordovician sedimentary environment in Zhuozi Mountain section and the depositional characteristics of event deposits in Zhuozi Mountain section were revealed through describing the field outcrop in detail, fine depicting the petrology features. Results: This paper ascertains the precise time of the dual structure boundary of the Ordovician sedimentary assemblages by the lights of the simultaneous sampling and getting the latest conodonts biostratigraphy of Zhuozi Mountain section, which corresponds to the location of the end of conodont zone Pygodus Anserinus, earliest Sandbian Stage, approaching the boundary of Middle—Upper Ordovician age (458.4 ± 0.9 Ma, ICG2014). This happens to have the same view on the time of prototype conversion of Ordos Basin. Comparative study indicates that the Yingtaogou Formation and its underlying stratum constitute a passive continental margin—flysch sequence structure. As a typical foreland basin sequence, it represents the corresponding strata during the formation of foreland basin during the Middle Ordovician time. Several sets of slump breccia in the flysch sequence of the Yingtaogou Formation and microscopic studies show that these slump breccias may be the result of the collapse from the eastern margin of Alxa Block (passive continental margin) during the Alxa Block(?) gradually close to the North China Craton. And the Ordovician dual structure of Zhuozi Mountain section corresponds to the the first two stages of Sinclair foreland basin formation model, which suggests Helan Mountain area was affected by the foreland basin at that time. Comprehensive analysis showed that the Ordovician tectonic regime of the northwest margin of the Ordos Basin changed from extension to aggregation, and this time limit of the transform may represent the initial collision between the Alxa Block(?) and the North China Block.Conclusions:The northeastern margin of the Ordos Basin probably constitute a foreland basin system formed during the collision between the Alxa Block(?) and the North China Block during the Ordovician. And this foreland basin system occurred at the early Sandbian Stage, Middle Ordovician, which may represent the initial collision between the Alxa Block(?) and the North China Block.