In recent 30 years, Th/U ratio has been using widely in evaluating the paleoredox conditions during the depositions of source rocks. However, there are few studies on the preconditions of the use of Th/U ratio for evaluating the paleoredox conditions. In many areas, the evaluation results of redox conditions in paleoenvironment obtained by Th/U ratio is different from that reflected by other parameters. The accuracy of judgment of paleoredox conditions by Th/U ratio is questioned. In this paper, the weathering degree of source rocks during the deposition are determined by chemical index of alteration (CIA) and Al₂O₃-CaO, NaO-K₂O (A-CN-K) ternary diagram, whereas the lithologies of parent rocks during source rocks depositions are determined by A-CN-K ternary diagram, Al₂O₃/TiO₂ ratio and REEs. Furthermore, the effects of the weathering degree, the properties of parent rocks and sedimentation rate on Th/U ratio to evaluate the accuracy of paleoredox conditions are analyzed. The results show that in lacustrine basins, although the content of uranium (U) gradually increased with the decrease of oxygen content in the water column, the obvious enrichment of thorium (Th) in sediments because of parent rock undergoing strong weathering and relative fast sedimentation rate influences the accuracy of Th/U ratio for evaluating the paleoredox conditions. Additionally, the different initial content of U, Th and Th/U ratio in different lithologies also influence the accuracy of Th/U ratio for evaluating the paleoredox conditions. The distribution characteristics of Th/U ratio are the result of the comprehensive influence of various conditions, not just paleoredox condition. When evaluating the redox condition of paleolake water by the enrichment factors of redox sensitive elements (RSEs) and bimetal ratio, the effect of weathering, lithologies of parent rocks and sedimentation rate should be taken into consideration on distributions of parameters.

近30年来，Th/U比值在评价烃源岩沉积古氧化还原条件方面得到广泛应用。然而，关于使用Th/U比评估古氧化还原条件的前提条件的研究很少。在许多地区，Th/U比值得到的古环境氧化还原条件评价结果与其他参数反映的结果不同。Th/U比值判断古氧化还原条件的准确性受到质疑。本文用化学蚀变指数(CIA)和Al ₂ O ₃ -CaO、NaO-K ₂来确定沉积过程中烃源岩的风化程度。O(A-CN-K)三元图，而烃源岩沉积过程中母岩的岩性由A-CN-K三元图确定，Al ₂ O ₃ /TiO ₂比率和 REE。进一步分析了风化程度、母岩性质和沉积速率对Th/U比的影响，以评价古氧化还原条件的准确性。结果表明，在湖盆中，虽然随着水体氧含量的降低，铀（U）含量逐渐增加，但由于母岩经历强烈风化和较快的沉积，沉积物中钍（Th）明显富集。速率影响评估古氧化还原条件的 Th/U 比的准确性。此外，不同岩性下U、Th初始含量和Th/U比值的不同也影响Th/U比值评价古氧化还原条件的准确性。Th/U比的分布特征是各种条件综合影响的结果，而不仅仅是古氧化还原条件。用氧化还原敏感元素（RSEs）富集因子和双金属比评价古湖水氧化还原条件时，应考虑风化、母岩岩性和沉积速率对参数分布的影响。