By utilizing measured pressure and mudstone sonic log interval, the distribution characteristics and evolution of the pressure of Bonan sag in Zhanhua depression were analyzed, and the overpressure mechanisms were studied. The results show that variation of measured fluid pressure with depth exhibits ＂two steps＂： When buried depth is lower than 2.35 kilometers, the formation pressure is basically shown as normal pressure; Low amplitude overpressure starts to appear between the depth of 2.5 kilometers and 3.0 kilometers ; The highest pressure coefficient can be as large as 1.8 between the depth of 3.0 kilometers and 4.0 kilometers, the Es3^z, Es3^ x and Es4s are the main formations where overpressure developed. The unbalance compaction and hydrocarbon generation are the main mechanisms of overpressure, and the regional difference exists. With the increase of depth in the middle step-fault zone and southern slop, the unbalance compaction phenomenon appears obviously and the range is corresponding to the first step; In deep sub-sag belt, over-pressured mudstone＇s density increases with the increase of the depth, but matches well with the range of the hydrocarbon generation. Hydrocarbon generation is the main pressuring mecha-nism in this zone and the depth range is corresponding to the second step. The amplitude of overpressure caused by unbalance compaction and hydrocarbon generation differs from each other, they both have their own pressure increasing quantified model which leads （o the appearance of the ＂two steps＂ characteristics of the formation pressure.
