There are abundant fossil plants in the Late Triassic Xujiahe Formation in northern Sichuan. According to the statistics of the percentages of thermophilic and hygrophilous plants in various members, the warm humid climate alternated with the arid (or semi-arid) climate throughout the deposition stage of the Xujiahe Formation. As shown by the chart of the vertical change of thermophilic and hygrophilous plants, the contents of thermophilic plants gets higher progressively and the content of hygrophilous plants gets lower from the first member to the fifth member of the Xujiahe Formation. Moisture increase and temperature decrease were favorable for coal formation. The deposition stages of the third and the fifth members were the best coal-forming periods. Two coal measures, three coal-forming stages, four kinds of sedimentary environments and three types of coal-forming plants may be distinguished on the basis of a study of the Xujiahe Formation.