Abstract: Located in northwestern part of the Sulige Gasfield, Block Su 53 has many horizontal wells drilled every year. The majority of horizontal wells drilled in the block target gas-bearing layers in Shihezi Formation, so their building-up sections often need to drill through mudstone in Shiqianfeng Formation and Shihezi Formation which are likely to have lost circulation and collapsing, with frequent downhole complications. Well Su 53-78-71H drilled in the block contains 5 boreholes formed in 4 side-tracking operations in the building-up sections.The anti-collision design and implementation of the No. 4 sidetracked borehole is especially difficult, which is unprecedented in the block. In this paper, selection of optimal side-tracking position, design of borehole curvature to be drilled, separation from existing boreholes of No. 4 borehole, and anti-collision design of other boreholes have been reviewed. Borehole trajectory control, anti-collision measures in directional drilling sections after side-tracking and accurate control over vertical landing depth in target formations have been studied to ensure safety of downhole operation and accurate landing into the target formation.