Abstract: There is only Mesozoic coal-bearing formation in Sandu mining area. The Early Jurassic Tanglong formation over main coalbearing strata situate in transition zone of fluvial deposits towardsmarine deposits of transgression sequence. The character of sedimentary sequence, lithology, fossils, grain size, microelements and bedform in Tanglong formation indicate double influence of stream and tidal in sedimentary environment. The marine water was intruted to north from sounthern Guangdong in the Late Triassic. When transgression was enlarged towards Hunan in the Early Jurassic,the mining area was developed branching bay in northwestern side of Big Xiangyuegan Embayment.