High Frequency Radio Frequency Identification(HF RFID) has been widely applied in different industry fields. Multiple tags are stacked together closely and can not be read normally in some practical cases. An electromagnetic model of stacked tags is proposed and the coupling of the tags is characterized. It is pointed that the frequency shift of multiple stacked tags may cause the stacked tags not to be read or written properly by mathematical analysis. A lower boundary of the frequency shift is given according to coupling coefficients. This work can provide a solid theoretical basis and new research direction for solving this bottleneck problem in industrial implementation.