Large carbides (> 20 µm) in 45Cr9Si3 valve steel can be effectively refined by promoting the annealing temperature to an appropriate region based on carbide determination. The carbides of electroslag remelting throughout the rolling and heat-treatment processes have been evaluated by applying electrolytic extraction, field-emission scanning electron microscopy (FESEM) analysis, automated inclusion analysis, and theoretical analysis by FactSage thermodynamic software, combined with laboratory-scale orthogonal experiments of heat treatment. First, the large carbides were extracted by a nondestructive nonaqueous solution extraction method, confirming the presence of mainly (Cr, Fe)7C3 and (Cr, Fe, Mo)7C3 carbides by X-ray diffraction (XRD) and phase analyses. The three-dimensional morphologies of the primary carbide particles were rod-like, and the maximum size was over 20 μm. Furthermore, it is implied that the annealing temperature was the most important factor affecting the carbide size. Finally, the optimized heat-treatment parameters for refining the carbide size of 45Cr9Si3 valve steel of the present strategy were obtained from laboratory-scale orthogonal experiments with water-cooling after annealing at 1050°C for 4 h and tempering treatment at 760°C for 2 h.