To solve the problem of lacking of spare well slots on offshore platform, the adding well slots technology is researched and developed. By analyzing and demonstrating the space condition, the drilling type, structure strength and system capabilities of the old platform, the applicability of the adding well slots was confirmed. The design of the adding well slots was refined and improved, and then it was carried out in the engineering practice.It was demonstrated that the adding well slots can meet the needs of drilling adjust well of old platform with the specialty of simple structure, short project period,low-cost and great economic benefits.