Abstract：During underbalance drilling gas reservoir is opened gradually，gas is in the shape of spherical from formation into annular flow and bubbles will be inflation migration，so radius of bubbles formation is important for the annular fluid。The study establish bubbles generated invasion of the diameter calculation model under different of gas kick, and given the rule of flow pattern. Analysis derived that underbalanced pressure drop and pore radius is an important factor of bubble diameter，When the pressure drop is constant， the bubble diameter will increasing as surface tension and pore radius increasing；While gas kick volume increases further，pressure drop becomes larger，the affect of fluid density and surface tension is little.