Intelligent reflecting surface （IRS） is one of the most attractive key techniques to realize smart radio environments. It is effective to mitigate Doppler effect in the high-mobility environments by deploying IRSs. An IRS-assisted Doppler mitigation method has been proposed for high-mobility communications in the literature. However， the computational complexity of IRS phase optimization is high due to the maximum likelihood estimator for partial channel parameters. A simplified IRS phase optimization method is proposed， and the phase expression is derived. The channel improved parameters can be obtained by a low-complexity channel estimation method. Compared with the other scheme， the new scheme avoids the complex estimation methods， prevents the additional estimation errors， and effectively reduces the computational complexity. Numerical simulation results show that the new scheme can effectively reduce the program running time， while still achieving superior passive beamforming gain and strong robustness when pilot overhead is limited.