Abstract: This paper discusses the principle and method of triaxial stress of casing string，analyzes the external loads applied on the casing string in detail，deduces the triaxial stress design from triaxial stress formula and external loads，and makes up the design program. Calcula-tion results show that this method is not only safe but also economical，and may be for field
technicians'reference.

Key words: Design of casing string Stress of C1Slilg Three-