Abstract: Rational drilling string is critical to successful drilling, especially in the extend reach well. In this paper, based on previousresearch, the drilling operation parameter and the hole trajectory of extended reach wells are taken into account comprehensively.Combined with the mechanical properties of the whole drilling string and its subassemblies, optimal design analysis is conducted on thewhole string and its subassemblies respectively. The optimization design method of drilling strings in extended reach wells is developed.A case study is made on the extend reach well A4 ERW3 in the Liuhua Oilfield in South China Sea, and the result is better coincided withthe test result. It is proved that the new method is of great significance to the optimal design of strings in other extended reach wells.