<p>Numerous highly coupled geometric errors (GEs) in five-axis tool grinders restrict the machining accuracy of high-performance complex tools. Due to their unclear quantitative influence mechanism on trajectory errors (TEs) during grinding, it is challenging to identify the most influential GEs for efficient targeted error compensation. Therefore, this paper proposes a key error identification method for tool grinders based on the GE-TE model and EFAST method. Firstly, the machine topological chain and GE distribution of the grinder are analyzed for spatial error modeling, and the GE-TE model is further established by combining the influence of spatial errors on TEs during the spiral groove grinding of a four-edge circular end mill. Then, a key error identification method combining the GE-TE model and the EFAST method is proposed for identifying the most influential GEs and components, where the GE-TE model ensures error model accuracy while the EFAST method improves error identification efficiency with a lower sampling requirement. Finally, comparison experiments based on the Sobol method and Morris method as well as error numerical correction (ENC) experiments, are conducted. Results show that the identified key GEs with the three methods are the same, and the principal error reduction rate after correction is up to 96.90%, proving the method’s effectiveness. It is conducive to the follow-up targeted error compensation and accuracy enhancement.</p>