The studies of sedimentology and geochemistry of two sorts of siliceous rocks, ie. the Lower(O1) and Upper (O2-3) Ordovician siliceous rocks from the Laohushan area, North Qilian orogenic belt, show the following different features: the Lower Ordovician siliceous rocks suggest a bio chemical origin, and were formed in a deep sea setting of the passive continental margin; the siliceous rocks intercalated with basalts  in the lower part of MiddleUpper Ordovician suggest a hydrothermal origin, and were deposited in an oceanic ridge setting; and the siliceous rocks in the upper part of MiddleUpper Ordovician suggest a biochemical origin,  and were deposited in an active continental margin in a deep  sea environment. Geochemical and sedimentological characteristics of the siliceous rocks imply the existence of a relatively steady setting of the passive continental margin in the Early Ordovician in Laohushan area, North Qilian orogenic belt. The siliceous rocks and terrestrial clastic rocks belong to the continental slope facies. In the MiddleLate Ordovician, the Qaidam  Plate was subducted under the North China Plate. As a result, an abyssal basin of the continental margin was formed in Laohushan area, and then a backarc basin was formed. The MiddleLate Ordovician siliceous rocks together  with pillow basalts as well as turbidites,are of  the backarc basin origin.