Post-rift abnormal subsidence have been discovered in Pearl River Mouth Basin (PRMB)，and during the study, the Baiyun Sag in the continental slope always is the focus, not the shelf of PRMB. The article figure out the lithospheric stretching factor and analyze the rule of post-break subsidence along the north of 1530 seismic line in the continental shelf of PRMB jointly by forward and reverse modeling using the flexural-cantilever model about stretching basin. It is recognized that the basin rifted from north to south gradually along the north of 1530 seismic line by forward modeling. The average shelf stretching factor is 1.2 and the total stretching factor range is 1.08~1.24 among the deeper sags. The fact that the value of practical post-break subsidence is 2.5km more than that of theoretical is revealed by comparing the value of practical and theoretical so the post-breakup abnormal subsidence may be the reason. That post-rift abnormal subsidence in the continental slope of PRMB can also be known through previous works and regarding to the cause of the abnormal subsidence on the continental shelf and slope, there must be some relationship between them, even among other continent margins of South China Sea. Maybe they are the outcome of a common mechanism. By analyzing the subsidence rate among the deeper sags, we found two episode of post-breakup subsidence from fast to slow exist. The rate are 140-190m/Ma between 30-23.8Ma and 36-65m/Ma between 23.8-18.5Ma. Then it is 300m/Ma between 18.5-16Ma and 75-110m/Ma since 16Ma. It is almost the same time with Baiyun sag in the slope which has its significant accelerated sedimentation rate at 17.5-15.5Ma. So maybe an important tectonic event occurred between 18.5-17.5Ma which can bring the fast subsidence across PRMB. The cause, time and range of the tectonic event still need to be studied.