The invention discloses a GRNN motorized spindle thermal error modeling method based on genetic algorithm optimization, and the method comprises the steps: building a four-layer generalized regression neural network (GRNN) structure, carrying out the global search of a generalized regression neural network smooth factor sigma through a genetic algorithm, and simply and accurately finding a global minimum value; initializing a generalized regression neural network smooth factor sigma by adopting a random population initialization mode, constructing a fitness function of a genetic algorithm (GA), calculating individual fitness, executing natural operation on a population, and selecting, crossing and inheriting individuals; establishing a GRNN framework, training samples, so population evolution gradually reaches the training precision, and finally, verifying the generalization of a generalized regression neural network (GA-GRNN) optimized by a genetic algorithm by adopting experimental data of different rotating speeds. According to the method, global optimal search is carried out on the smooth factor sigma of the GRNN by using the genetic algorithm, and the prediction precision and generalization ability of the GRNN are improved.