Using flexural backstripping and gravity anomaly modeling techniques and Process-Oriented Gravity Modelling (POGM), we calculated variable effective elastic thickness (T_e) of a long cable, multi-channel seismic reflection profile across the northern margin of the South China Sea and analyzed the lithospheric strength distribution during rifting and post rifting. Results show that the lithospheric strength is weak during rifting and that the post-rifting T_e is different for different tectonic units. The lithospheric strengths of the Panyu Low Rise and Lower Slope are larger, with T_e of about 15 km; the T_e of the Northern Depression is about 7 km, and the Baiyun Sag is the weakest in strength, whose T_e is only about 5 km. Lithospheric strength can help to understand the distribution of the basin and the process of lithospheric evolution in the northern continental margin of the South China Sea.