Abstract: The age of the Sunjiagou Formation （i. e. Shiqianfeng Formation in a narrow sense） has been long disputed. This problem influences the determination of the age of the Shiqianfeng Group and the Paleozoic-Mesozoic boundary in the whole North China platform region. The author has studied the marine fossil orizon of the Shiqianfeng Group in Weibei, the stratigraphic position andage of the Shiqianfeng Group in eattern and western Henan and the regression-type diachronism of the Upoer Paleozoic in North China. According to these studies, he proposes the new recognition that the Sunjiagou Formation is diachronous strata, i. e. its southern part is of early Early Triassic ago because of relatively late sea regrettion, while its middle and northern parts are late Late Permian in age.