Abstract: The tensile mechanical model is established, which includes "parallel length system" and "testing machine system". According to this model, when the compliance (reciprocal of stiffness) of the testing machine system is considered, the general expression of the crosshead displacement rate is derived. When the additional elongation component of the testing machine system is directly considered, another general expression of the crosshead displacement rate with different forms is derived. By using the above general expressions, the relationship between the two kinds of crosshead displacement rate is established, and the "slope compensation" crosshead displacement rate formula and the "K value compensation" crosshead displacement rate formula are obtained respectively. It is proved that the two formulas are equivalent to each other. This paper points out the problems existing in Appendix F of current international standard ISO 6892-1:2019 and puts forward some suggestions for revision.