Abstract: Linear gyrokinetic simulations have been performed using the gyrokinetic toroidal code (GTC) code to investigate the effects of fast ions on the dominant instability changes from trapped electron mode (TEM) to ion temperature gradient (ITG) in the EAST. The effects of fractions (5%, 7.5% and 10%) and temperatures (10 keV, 20 keV and 30 keV) of fast ions are investigated. The dominant mode transitions from TEM to ITG mode under different plasma parameters. A transition of the dominant instability is observed at 0.7 \leqslant k_\theta \rho_\rms \leqslant 0.8 and shifts to k_\theta \rho_\rms \approx 0.7 when fast ions are considered. In addition, fast ions stabilize ITG modes, and the effect becomes more significant as the fraction of fast ions increases, reducing the growth rate by approximately 10% when the fraction of fast ions is 10%. The stabilizing effect increases with increasing temperature of fast ions, but varies little for T_\textf \gt 20\; \rmkeV . Fast ions reduce the ion temperature gradient R/L_T_\texti threshold of the dominant instability changes from TEM to ITG. The threshold decreases with decreasing temperature of fast ions, with a reduction of about 10\%-20\% when the fraction reaches 10\% in hydrogen plasmas, and about 3\%-12\% when deuterium is the main ion. Furthermore, the normalized electron temperature gradient R/L_T_\texte for the dominant instability changes from ITG to TEM is lower in hydrogen plasmas, decreasing from 5.4 to 4.1 as T_\textf increases from 10 keV to 30 keV, while in deuterium plasmas it decreases from 10.5 to 9.5. Moreover, electron-electron collisions lead to the dominant instability changes from TEM to ITG at 0.4 \leqslant k_\theta \rho_\texts \leqslant 0.8 and strongly stabilize the ITG mode, reducing its growth rate about 50\% , while ion–ion collisions have little influence on TEM and ITG.