Abstract: In order to deal with the loss of circulation caused by extension of horizontal section,Managed pressure drilling was applied to keep downhole circulating pressure under leak-off pressure during drilling.Allowable length of horizontal hole section with 90° inclination was calculated based on leak-off pressure,downhole circulating pressure and annular pressure loss in each meter of hole.If above length can’t meet the requirements of development,the longest horizontal length and inclination angle are calculated for safe drilling according to the thickness of pay zone,depth of oil-water contact ,circulating pressure above pay zone,and annular pressure loss for each meter of hole.It was predicted that horizontal well length for safety drilling was only 965 m for Well JZ25-1S-17 in JZ25-1S Oilfield.To enhance the well yield,it was estimated that length of hole could be extended in pay zone with less than 86.67° inclination.Actually with the well inclination controlled under 86.67°,the well length in pay zone reached to 1068 m without fluid loss.It showed that the method could effectively prevent the loss of circulation caused by extension of horizontal section while drilling.