Penglaiba section of the Kalpin area, Tarim Basin, serves as a typical section for Cambrian research, whereas no detailed carbon isotopic research on the Cambrian Series 2—Series 3 boundary has been documented up to date. Carbon isotope analyses were carried out on the carbonate rocks of Cambrian Series 2 (Wusongger Formation)—Series 3 (Shayilik Formation) transition in the Penglaiba section. The δ13C values (VPDB) vary between -4.2‰ and 0.8‰, while the δ18O values (VPDB) vary between -7.4‰ and -3.8‰. Moreover, there is no obvious correlation between the δ13C and δ18O values， indicating the δ13C values recorded a memory of ancient seawater chemistry. A large negative δ13C excursion up to -4.2‰ at the boundary between the Wusongger and Shayilik formations characterize the Cambrian Series 2—Series 3 boundary. This negative δ13C excursion could be correlated with the ROECE events reported for Laurentia, Siberia, North China and South China. As ROECE happened at approximately 509 Ma, our report offers a reliable age for the Cambrian strata in the Tarim Basin. This work is also the first report of ROECE excursion in the Tarim Basin, and highlights the great potential of δ13C variation in Cambrian subdivision and global correlation.