Abstract: Abstract：The assemblage characteristics, distribution and paleoecology of ostracods in the Fuyu and Yangdachengzi oil horizons in the Sanzhao area, Songliao basin, have been studied systematically. Study shows that ostracods appeared in small numbers in the early stage of the deposition of the Fuyu and Yangdachengzi oil horizons, then developed and thrived, and were all extinguished in the end of the depositional period. The above change is not only manifested by the significant increase in numbers of genera and species of ostracod assemblages, but also by their regular changes in shell ornamentations, distribution and preservation state. In general, these ostracods are characterized by simple shell ornamentations, a high diversity and mixing of individuals of varying sizes and living in a shallow-water lake environment. Further analysis of the burial environment and distribution range of ostracods shows that an alluvial plain appeared during the period of deposition of the Yang-4 and Yang-5 formations that only contain small quantities of ostracod fragments, while the depositional environment in which the Yang-3 Formation and Fu-Upper Formation containing relatively abundant ostracods were deposited was mainly a shallow lake one. After a large shallow lake appeared in the central part of the area during the deposition of the Yang-3 Formation, the lake gradually increased in area and depth with time and expanded westward to the west of Changyuan of Daqing during the deposition of the Fu-Lower Formation, and as a result, it was connected with another lake in the Qijia-Gulong area to form the unifying Songliao paleolake. During the period from the deposition of the Yang-3 Formation to the deposition of the Fu-Upper Formation, the shallow waters, dry climate and strong oxidation resulted in a red color of lacustrine sand and mud in the Sanzhao Area.