The Mesoproterozoic Wumishan Formation in Beijing area is mainly composed of chert banded dolostone, chert nodule dolostone, stromatolite dolostone and a small amount of terrigenous silty clastic dolostone. It is widely exposed in the Beijing mountain area with a thickness of 2000~3500 meters. The provenance and cause of the formation of a large number of Mesoproterozoic siliceous sediments represented by the Wumishan Formation have been a controversial issue in the geoscience field for a long time. Most researchers believe that layered chert is biogenic and nodular chert is the diagenetic replacement. After detailed field investigations in the Western Hills of Beijing and other places, the authors have found that most of the siliceous bands or siliceous nodules (generally referred to as chert mat or siliceous gel mat in the following) occur on the sedimentary discontinuity surface, especially on the ancient erosion surface. The underlying dolostone gravels are often cemented with the siliceous material. The interior of chert mats is often mixed with impurities, especially organic matter, showing various dark and mottled colors. Chert mats were formed at the same time with surrounding dolomitic debris before the accumulation of the overlying dolomitic sediments. There is a relationship of mutual interpenetration and wrapping between chert mats and dolomitic sediments, but the boundaries between them are clear. Neither dolostone debris nor chert mat has obvious replacement trace. Therefore, the author believes that the “concretion” of chert is the result of silica gel aggregation and solidification. Silica gel solidification is the only genetic mechanism of primary chert in Wumishan Formation. Usually, the cementation rate of dolomitic mud and intraclast is obviously faster than that of silica gel mat. When an earthquake occurs or the overlying sediment is too thick, the soft silica gel will intrude upward or squeeze downward along the weakly consolidated dolostone fissure to form siliceous dykes. In the event of later magma intrusion or transformation by thermal metamorphism, the composition and color of chert strips and chert concretions will change accordingly. The texture becomes pure, and the color changes from dark to light, but the original bedding or texture is still retained, which is easily mistaken for diagenetic or post-diagenetic siliceous replacement of carbonate minerals.