Abstract: Field geological surveys and integrated researches indicate that the main horizon of the Jehol biota containing rare fossils such as the reptile Sinosauropterxy and the angiosperm Archaefructus in the Yixian Formation is fossiliferous lacustrine sedimentary strata of the Second Member of the Yixian Formation be-tween the4th and5th basic-intermediate volcanic cycles.Study of the data about sporopollen,plant and wood fossils in sediments from different localities and analysis the paleoclimate and paleoenvironment reflected by them show that large numbers of hygrophilous and thermophilous plants grew during the Yixianian age,generally indicating a warm and wet habitat.The common occurrence of xerophilous Gnetales and features of Bennettiales with membranous leaves and Conifersles with scaled leaves indicate the possible occurrence of seasonal arid or semi-arid climatic conditions ,which may be associated with volcanic activity.