Abstract: Ballooning is a term often used to describe the physical phenomena where drilling fluid is lost while circulating and then regained when the pumps are shut down. In deep water drilling, due to the narrow window between pore pressure and fracture pres-sure of formation, and weakness, when equivalent circulating mud density is close to the formation fracture pressure, more than equivalent mud density of closing induced or in-situ micro fractures, formation ballooning can be observed. It is difficult to deter-mine formation ballooning, or real kick, should circulate out the bottoms to confirm it by the driller's method, which adds additional time and risk in well control operation. In this paper, the mechanism of formation ballooning has been analyzed, and the methods to identify a formation ballooning has been discussed, and the measures for dealing with this problem have been put forward, which can provide reference for deep water drilling.