There are a series of drilling technical challenges in Y oilfield horizontal wells of Iran, complex geological lithology, uncertain geological target area, long uncased hole section, big borehole friction and difficult trajectory control of long horizontal well section. Based on the practice in the first phase horizontal well drilling in this oilfield, 6 drilling technologies for horizontal well are summarized and formed: optimization of casing program, screw drill tool types and the optimization of stabilizer outer diameter, bit selection, bottom hole assembly (BHA) for sidetracking, optimized and quick drilling in horizontal well section and KCl polysulfonate drilling fluid. These technologies are fruitfully applied in well YS9, the construction of directional and horizontal sections were completed 12.4 days ahead of schedule with shorter well construction cycle and reduced cost.