The initial solidification process of microalloyed steels was simulated using a confocal scanning laser microscope, and the growth behavior of austenite grain was observed in situ. The method for measuring the initial austenite grain size was studied, and the M0*(the parameter to describe the grain boundary migration) values at different cooling rates were then calculated using the initial austenite grain size and the final grain size. Next, a newly modified model for predicting the austenite grain size was established by introducing the relationship between M0* and the cooling rate, and the value calculated from the modified model closely corresponds to the measured value, with average relative error being less than 5％. Further, the relationship between Tγ (the starting temperature for austenite grain growth) and equivalent carbon content CP (CP>0.18％) was obtained by in situ observation, and it was introduced into the modified model, which expanded the application scope of the model. Taking the continuous casting slab produced by a steel plant as the experimental object, the modified austenite grain size prediction model was used to predict the austenite grain size at different depths of oscillation mark on the surface of slab, and the predicted value was in good agreement with the actual measured value.