Abstract: The unconsolidated Neogene shallow sandstone in Enping Sag of Pearl River Mouth Basin causes severe sand production in the development process and brings great difficulties to the efficient exploitation of oil fields. Through the study of sedimentary origin, diagenesis and oil-gas charging period of Hanjiang Formation in M Oilfield of Enping Sag, the paper reveals the main controlling factors of unconsolidated sandstone, which lays a foundation for subsequent production measures such as well location adjustment according to attributes. The results show that there are three controlling factors contributed to the unconsolidated sandstone as follows: 1) Hanjiang Formation in the study area is of delta front sedimentary environment with fine grain size and high mud content, which is not conducive to cementation; 2) the diagenetic degree is low, belonging to stage A of early diagenesis, with poor cementation; and 3) the diagenesis is severely inhibited by oil-gas charging.