This paper is the result of geological survey engineering. Objective The catastrophe and extinctions that occurred in Late Cretaceous is during geological history make the K/Pg boundary become one of the most important geological boundaries in the world. The study of Late Cretaceous–Early Paleocene sedimentary environment evolution of Tarim is helpful to reconstruct the paleogeography in the northern Tethys basins. Methods This study employs the sensitivity index analysis of geochemical elements from core samples of Well PBX1 located in the southwestern Tarim Basin to investigate the environment evolution associated with the K/Pg boundary. Results The climate was hot and dry in Late Cretaceous and transitioned to a short-term warm and humidity environment in Early Paleocene. The paleo-salinity changed from normal marine environment in Late Cretaceous to brackish water or marine to saline lake in Early Paleocene. The salinity reached minimum at the K/Pg boundary. For redox environment, it experienced the oxic to anoxic environment and weak redox environment in Late Cretaceous to a short-term oxic environment near the boundary to a major anoxic and weak redox environment in Early Paleocene. Conclusions The geochemical element sensitivity index of well PBX1 reveals the response characteristics of the catastrophic anomaly event between Cretaceous and Paleocene in the Tarim Basin, which is effective evidence of the of Late Cretaceous–Early Paleocene sedimentary environment evolution in the northern basins of Tethys Ocean.