Abstract: As the second longest river in China, the formation and evolution of the Yellow River is of great significance to the evolution of geomorphic pattern and paleoclimate change in China. Since the 19th century, scholars have put forward a hypothesis that the Yellow River once entered the sea from Hetao Basin to the east through the Yongding River before entering the sea along the Sanmen Gorge. This imaginary river can be called "North proto-Yellow River" and has wide influence in the field of geography. However, there has been little evidence to definitively confirm or disprove it. In this paper, this hypothesis was discussed based on recently published data of the boreholes in the North China Plain, the sedimentary evidence in the Fenwei Basin, and the geomorphic evidence in the Sanmen Gorge. These lines of evidence show that no materials from the Upper and Middle reaches of the Yellow River had been transported to these boreholes located in the alluvial fan of the Yongding River and along the coast of Bohai Bay before 1.6 Ma, indicating that the Sanmen Gorge and the North proto-Yellow River were not opening at this time. The materials from the Upper and Middle reaches of the Yellow River began to enter the Bohai Bay after 1.6 Ma, and the alluvial fan of the Yongding River still lacked materials from the Upper and Middle reaches of the Yellow River. Combined with the sedimentary evidence from the Sanmen Formation in the Fenwei Basin where the lacustrine depositions rapidly ended and changed to loess depositions at about 1.6 Ma, it could be concluded that the Yellow River was not yet fully connected before 1.6 Ma, and it entered the North China Plain through Sanmen Gorge, and thus the Yellow River never flowed into the sea through the Yongding River.