The platform adding well slot can make full use of the old platform workover rig and other equipment resources, the construction period is short, the investment cost is low. In recent years, it has been widely used in engineering. Taking the PL9-3WHPB platform external-addition well slot project as an example, this paper introduces in detail the location scheme comparison and selection of the external well slot, the design of the wellhead piece, the overall safety evaluation of the platform and the installation and maintenance of the wellhead, analyzes and summarizes the problems encountered in the external well slot and the solutions, and provides reference and ideas for similar adding well slot projects.