Offshore airborne gravity plays very important role in the unifying of the land geoid and marine geoid. Airborne gravimetry using Chinese airborne gravimetry system is flown in Bohai bay in recently three years, which the surveying area extends about two hundred thousand square kilometers and 5??5? grid mean gravity anomaly has been obtained. The gravimetry is summarized and its key processing method is introduced firstly. Then, we discuss the accuracy estimation methods for the airborne gravity in detail, and introduce so-called grid-overlapping method to appropriately evaluate the internal accuracy of the grid mean gravity anomaly. The results indicate that the standard derivation of the crossover errors is about 1.5mGa1 at the resolution of 5 arcmin, the standard deviation of the 5??5? grid mean gravity anomaly evaluated by grid-overlapping method is about 1.6mGal, and the accuracy of the 5??5? grid mean gravity anomaly are both better than 3.0mGa1 compared with the satellite altimetry gravity data and the seaborne gravity data respectively. The systemic error and the standard deviation of the 1??1? mean airborne gravity compared with the GOCE gravity field model are less than 0.5mGal and about 2.7mGal respectively. The accuracy of the regional geoid using airborne gravity data in the surveying area is improved to 12cm, whereas it is about 23cm computed from EGM2008 model.