Abstract: Ma 42 flat 2 well,the first well designed and accomplished independently by Huabei petroleum administration bureau for developing oil and gas reservoir in buried hill formation,is a slim-hole sidetracking horizontal well with window sidetracking in ?177.8 mm casing pipe.Whipstock is put down in dense formation of Changlongshan group in Qingbaikou system,entering buried-hill formation by milling cone window sidetracking,oblique reducing,stabilizing and increasing of sidetracking.?152 mm whipstock is chosen reasonably in construction cooperated with wireless inclinometer,which brings one-time seating and sealing success.Gabby cemented carbide is optimized as milling cone window sidetracking in ?154 mm XZ series of main cutting edge,which brings one-time success.Vibration of hole trajectory is mastered in time by wireless inclinometer and control of sidetracking hole trajectory is ensured by using small hole guiding motor of different specification,which improve the coincidence rate of hole profile and hitting target accuracy and achieve drilling of high quality,high speed and safe.The window sidetracking success of ?177.8 mm horizontal well provides favorable reference and reliable technical guarantee for development and application of small hole in future.