Abstract: The composition and characteristic parameters of rare earth elements in sediments provided sedimentary environment as well as fluid and source information, which can be used to judge the sedimentary environment of source rocks and reservoirs and the sources of oil and gas. The tests of rare earth elements in carbonate rocks from the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin indicated that the total REE content was low. The LREE and HREE differentiation were obvious. Obvious negative δCe and positive δEu anomalies were observed. The low total REE content and negative δCe anomaly reflected a relatively open and weakly oxidizing low-energy environment when the fourth member of the Middle Triassic Leikoupo Formation was deposited. This environment was favorable for the breeding and growth of hydrocarbon generation organisms, hence providing a material basis for the formation of hydrocarbon source rocks in the Middle Triassic Leikoupo Formation. The positive δEu anomaly suggested a strongly reducing environment for fluid activity during the late period, which was favorable for organic matter preservation. Enriched LREE, poor HREE, positive δEu anomaly and a REE distribution pattern similar to hydrothermal fluid indicated that the fourth member of Leikoupo Formation experienced late hydrothermal alteration. Hydrothermal migration channels also provided good permeability for deep oil and gas migration.