The pelagic barite precipitation in the seawater column is tightly associated with the biological pump, and thus the barite Ba isotope composition (δ138/134Ba) has been advocated as a novel paleo-productivity proxy. However, pelagic barites in sediments are susceptible to diagenetic processes that may modify primary δ138/134Ba signals and hamper the application. To constrain the diagenetic effect, we report δ138/134Ba data of diagenetic barites from the black shales of the lower Cambrian Yurtus Formation in the Aksu area of the Tarim Basin. The δ138/134Ba values vary within a large range from −0.15‰ to +0.49‰ and show a generally positive correlation with the 87Sr/86Sr ratios, indicating the mixing of two end-members. The samples with 87Sr/86Sr ratios close to the coeval seawater value record relatively low δ138/134Ba values from −0.15‰ to +0.14‰, which are inferred to inherit from the primary barites; whereas the samples with 87Sr/86Sr ratios exceeding the coeval seawater value record higher δ138/134Ba values from −0.05‰ to +0.49‰, which may have resulted from the alteration by the host black shales during early diageneses. Based on these investigations, we propose that pore waters below the sulfate-methane transition zone may provide an additional source of Ba (enriched in heavy isotopes) for the formation of diagenetic barites. We suggest to combine barite 87Sr/86Sr ratio with δ138/134Ba value to evaluate diagenetic influence on primary Ba isotope signatures especially for an over-determined system, which is important for applying Ba isotope compositions of barites to understand paleo-oceanic evolution.