Selection of cutting carrying medium is very important for unbalanced drilling, which should protect oil & gas reservoir and keep the borehole stability. Oil-in-water system was firstly applied in the fourth unbalanced drilling section of Well Lishen 1. Optimized drilling fluid formula and scientific maintenance played well in oil & gas formation protection and borehole collapsing prevention, a new type of cutting carrying medium was developed for unbalanced drilling construction in this area.