Abstract: The Xijiang Main Sag in the Pearl River Mouth Basin contains fuzzy areas in vertical reflection imaging, and the geological origin of the fuzzy areas is often related to the basin diapiric activity. Having studied the seismic profile and seismic impedance of the vertical fuzzy area in the Paleogene seismic data from the 33W Subsag in the Xijiang Main Sag in combination with the development and evolution of the basin, we believed that the fuzzy area has a dual structure with different deep-and-shallow reflections indicating a mud-fluid diapir structure. The Paleogene Wenchang Formation in the Xijiang Main Sag is a deep lacustrine facies widely distributed as mud source-materials for mud fluid diapir. A high-pressure anomaly was detected in the fourth member of the Enping Formation at the bottom of the 33W Subsag drilling well. The Xijiang Main Sag has the overpressure conditions to form a seal box that is covered by a widely-developed mudstone mudstone at the top of the Enping Formation. The magma intrusion in the Zhuhai period destroyed the sealing, activated mud fluid diapir in the basin, and formed a deeply-buried and weakly punctured conical mud fluid diapir in the Paleogene strata of the 33W Subsag.