The experiments were carried out to study the coupling characteristics of the I-port antenna in the ion cyclotron range of frequencies (ICRF) in EAST. The dependencies of the coupling resistance on various parameters including the antenna position, the central electron density, gas injection and the antenna phasing have been studied. The results obtained show that the experimental data are consistent with theoretical simulations. We find that the antenna loading decreases sharply at the L-H transition due to the change of the plasma density profiles in the scrape-off layer (SOL). The effect of the low hybrid wave (LHW) on the ICRF coupling during the H-mode is observed. The theoretical interpretation of the results is discussed, together with the efficient methods to optimize the coupling efficiency.