Abstract: Research on well completion for horizontal wells is developed to improve single-well output of S Oilfield in Kazakhstan. Joshi Formula is used to calculate change trend of oil well capacity in different lengths of horizontal well segments, and length range of horizontal well segments is optimized. Borehole stability of horizontal wells is under integrated assessment by making use of sand production rate, interval transit time of reservoirs, porosity and other parameters, and proper sand control process is necessary for horizontal wells of S Oilfield. It is considered upon analysis of reservoir temperature, clay content and other factors as well as the current situation of sand control and well completion methods for the existing horizontal wells that chemical sand control method is not suitable for horizontal wells of S Oilfield, and only the sand control method of slotted sieve tubes can be adopted Optimization design is conducted for slot width of screen tubes in combination with sand sample size distribution analysis results for representative wells of S Oilfield, and final width of sieve tube slot is determined to be 0.6 mm. The designed well completion process for horizontal wells is put in field application for 4 wells of S Oilfield. The output of S-431H and horizontal well S-431H reaches 2.5-3 times of that of vertical wells, and the output of S-615H and horizontal well S-615H even reaches 8 times to 10 times oilfield, showing a sound yield-increasing effect.