Abstract: Selective cementing technology has been used to exploit the carbonate reservoir, which has satisfied the needs of multistage reform and later stimulation of reservoir. External Casing Packer (ECP), which is the key tool to selective cementing technology, is discussed in this paper to explore its main failure causes during the selective cementing operation. Analysis of the bottom hole temperature, running-in influencing factors, sealing position diameter and pipe string structure shows that rubber tear strength reduced by higher bottom temperature, unsmooth running-in influenced by oversized hole curvature, intensified friction during running-in induced by incompetent drifting measure, loading capacity reduced by oversized sealing position diameter and unreasonable pipe string structures, and non-effective protection to the ECP are the main causes of the ECP failure. According to the failure causes above, corresponding technical measures have been established: the actual maximum operating temperature is rated operating temperature multiplying by a safety factor; using the effective technical measures to control the borehole curvature; simulating drifting with the watermelon mill; choosing the rational borehole diameter of the sealing up position and connecting short casing fitted with centralizer at both ends in the ECP. These measures are significant for reducing the possibility of ECP failure and improving the safety reliability of selective cementing technology.