The Early Carboniferous Visean plant fossils are reported for the first time from the upper part of the Ta′erbahatai Formation in northwestern Junggar，including Archaeocalamites scrobiculatus，Sublepidodendron grabaui，S. mirabile，S. cf. mirabile，Sublepidodendron sp.， Lepidodendropsis sp.， Mesocalamites sp., Sigillaria sp., Rhodeopteridium ?sp. and Knorria sp.. This floral assemblage demonstrates that the age of the Ta′erbahatai Formation can be extended to the Early Carboniferous. The uppermost Ta′erbahatai Formation and its overlaying Heishantou Formation are obviously diachronous on the basis of fossil evidences and contact relationships of strata. The age of the uppermost Ta′erbahatai Formation extends from the Tournaisian to Visean and the Heishantou Formation is from the Tournaisian to Serpukhovian. Deep-sea trace fossils preserved together with plant fossils indicate bathyal-abyssal environment in the Tarbgatay area during the early-middle Early Carboniferous，in contrast to the littoral and shallow marine conditions in the Jimunai area at the same time. The different sedimentary systems are largely attributed to the extensive transgression in northern Xinjiang and the subduction of the Irtysh-Zaysan Ocean Plate under the Kazakhstan Plate during the Early Carboniferous，which also account for the diachroneity of the uppermost Ta′erbahatai Formation and Heishantou Formation in northwestern Junggar.