The invention belongs to the analysis field of quantitative metallography on all-form crystal grains in a steel material microstructure and particularly relates to an automatic measurement and fine classification method for steel crystal grains, and a system thereof. According to the method, an image acquisition device acquires the original images of to-be-measured crystal grains of the steel material firstly, and then the original images are pre-processed by an image pre-processing module. The pre-processed images are subjected to the region labeling treatment by an automatic measurement module, and then the images of to-be-measured crystal grains can be obtained. After that, the geometry characteristic parameters of the images of to-be-measured crystal grains are extracted, and then the characteristic morphological parameters of target crystal grains are measured through the random field area algorithm. The area of crystal grains is obtained, and then the grain size of crystal grains and the mixed crystal degree (GME) of crystal grains can be figured out. The mixed crystal degree (GME) of crystal grains is automatically classified by an automatic classification module according to a most suitable threshold. In this way, the blank in measuring and classifying the mixed crystal degree of steel crystal grains in the prior art can be filled up. Meanwhile, the characterization precision of the images of steel crystal grains is up to plus/minus 0.001 [mu]m. Therefore, by adopting the above method and the above system, the characterization precision of the images of steel crystal grains is highest during the steel metallographic structure analysis process.