Abstract: The development of offshore oil and gas is usually by way of cluster wells. Due to the particularity of offshore oil field development, most of the riser in the slot is driven in by hammering, which will certainly cause deformation or damage to some risers and cause deformation of the slot. Meanwhile, being limited by the riser strength, depth into the soil, etc., the damaged slot shall usually not be used again. So in order to solve this problem and realize efficient use of deformed and damaged slots, this paper took the deformed Φ508 mm riser on WHPA platform in Jinzhou 25-1 oilfield as an example and successfully carried out window sidetracking through riser by optimally selecting the Smith window cutting tools, strengthening the riser, reasonably controlling weight on bit, optimally selecting the window point and optimizing the trajectory control. This supporting sidetracking drilling technology was first used in China, and its successful experiences provide reference to the reuse of deformed and damaged risers in other platforms, and so it enjoys a favorable application prospect.