Based on the measuring data of landforms, high-resolution seismic profiles, drilling cores, etc, a diapir body was found in the north of the modem Yellow River delta. The diapir body with a length of 5 km and a width of 1 km is distributed on the middle to low part of the slope of an abandoned delta lobe. Its formation is related to the deformity of the soft stratum which is deposited in the prodelta and covered by the stratum of the mouth bar sediments. Research results show that its formation is very different from the Mississippi River delta’s diapirs but related to the erosion of the seabed and occurs on the location with a large eroded quantity. The soft stratum and its diapir body can result in a great hazard to marine buildings.