In the Beibu Gulf area, some of the riser pipe shoes are deformed, and the abandoned well slots cannot be used under the conventional drilling technology. This paper demonstrates the feasibility of window opening of the riser from the aspects of the stability of the riser, the bearing capacity of pile and soil, and the strength of the window. The influence of the depth of different window opening points of the riser on the bearing capacity of pile-soil is analyzed, the finite element analysis mechanical model of the riser window opening is established, the axial compressive strength and bending strength of the riser after the window opening are calculated, and the influence of different window opening depth of the riser and the deflection of the riser on the critical load of the wellhead is analyzed. The results show that the allowable axial force at the opening window of the riser decreases linearly with the increase of bending moment, and the strength at the window is the main factor to determine the allowable load at the wellhead. The closer the window opening point is to the mudline, the smaller the allowable load of the wellhead. The deflection of the lower part of the riser will increase the bending moment at the window opening, thus reducing the allowable load of the wellhead. When the depth of the window opening point is more than 20 m from the mudline, the depth of the window opening point and the deflection of the riser will have little impact on the allowable load of the wellhead. In the west of the South China Sea, it is the first time to realize the sidetrack drilling technology of the riser above the mudline to effectively use the abandoned well slot, which has reference significance for the reuse of the subsequent abandoned slot.