DP4 well is a horizontal one, locating at the Daniudi gas field of Ordos Basin and drilled with aerated underbalanced drilling technology. The target formation is He 1 with lower formation pressure coefficient from 0.83 to 0.99(average 0.93). To effectively protect He 1 formation, aerated underbalanced drilling technology was implemented in the whole horizontal section with aerated drilling fluid, and the protection was successful. The paper mainly introduced the aerated underbalanced drilling fluid about the technique & technology, implemetation process and the field situation; and also analyzed and evaluated implemetation effect of aerated underbalanced drilling fluid in this well.