Sidetracking in φ177.8mm case is one of the main development directions for vertical wells in old Tahe oilfield in recent years. Sidetracking in carboniferous kalashayi formation, iron scraps accumulate in the case opening because of large circulating pressure loss and narrow window for upward iron scraps, which bring high pump pressure, tangle cutter and difficult cutter backstroking; formation compactibility makes low penetration rate and blocking; penetrating 2 different pressure systems of carboniferous and ordovician causes lost circulation, large torque and drag. By the optimization of bit, drilling parameter, steerable motor and drilling tool assembly and by the trajectory monitoring and mud technique, sidetracking was successfully completed for old well reconstruction in Tahe oilfield.