Abstract: By using the Kramers-Henneberger (KH) transformation, the time-dependent Dirac equation in high-frequency (HF) intense laser fields can be changed to a time-independent equation with an effective Coulomb potential, which plays an important role in studying the adiabatic atomic stability problem in high-frequency laser fields. With numerical method, we have investigated in detail the characteristics of the obtained effective Coulomb potential. It has been found that when the laser intensity is high enough, the relativistic effect is so important that it modifies the effective potential drastically. Moreover, the dipole approximation, which is often used in literature when solving Schrodinger equation, is not proper for high-frequency laser fields.