The Lower Carboniferous strata consisting of the Xibeikulasi, Baogutu and Tailegula formations are widely distributed in the Baogutu area in West Junggar. But their geological age and stratigraphic sequence have long been controversial. This study carried out the petrology and zircon U- Pb geochronology investigation on tuffaceous conglomerate in the Xibeikulasi Formation were studied in order to provide some basis for the age and sequence of Lower Carboniferous strata. Petrographical analysis shows that the conglomerate in tuffaceous conglomerate accounts for 45% and is composed of sub- angular siliceous rock（2~15 mm）, crystal tuff, and tuff sandstone, and a small amount of limestone, cemented by crystal- bearing volcanic ash; fine- grained detrital rock is mainly fragments, feldspar and minor quartz and occurs in angular and sub- angular shapes, with grain size of 0. 05~0. 5 mm; and filling material is mainly volcanic ash. Zircon U- Pb dating reveals that the 206Pb/238U ages of the zircons vary mainly in the range of 327~382 Ma. The youngest peak at 330~335 Ma may represent the age of volcanic activity coincidence with the deposition of tuffaceous conglomerate, and the obvious peak at 345~360 Ma may represent the crystallization ages of most volcanic rock gravels in tuffaceous conglomerate. Several sets of ages are around 411~428 Ma, ~455 Ma and 482~504 Ma, which correspond to the ages of the Keramay, Darbut and Tangbale ophiolite mélanges, respectively. Our study shows that the deposition age of the Xibeikulasi Formation is Early Carboniferous Visean. Combined with previous researches, it can concluded that the probable sequences of these three strata is Xibeikulasi, Baogutu, and Tailegula Formation from bottom to top.