Readability of UHF RFID tags in dense environment will be influenced by mutual coupling effect between tags. The paper study mutual coupling effect from the perspective of frequency shift characteristic of antenna impedance. First, based on the structure and circuit model of inductive coupling tag antenna, the frequency characteristic of tag antenna impedance with meanderline is analyzed, in consideration of mutual coupling. Then the simulation and calculation verify the conclusion that antenna impedance shifts to lower frequencies. Combined with theory and simulation data, frequency characteristics of the mutual coupling effect is analyzed. It is put forward that the power transmission coefficient also shifts to lower frequencies, resulting in unreliable reading performance. The conclusion has guiding significance for research on the performance of the dense tags.