Abstract: Casing thread connection must satisfy structural integrity. Thus the thread off failure's influential parameters of casing thread connection were simulated by the finite element method that can be used to consider large distortion and large slippage on thread contact boundary Results show:thread taper has very small effect on thread off, and material property has the biggest effect on thread off. The first position of thread off is coupling end, and that position is tallied with the position during actual operation.