A method for designing a gear forging die by three-order non-uniform rational B-splines includes: determining feature points K1, K2, K3 and K4 representing symmetrical basic shape of a gear forging shaft, and using the feature points as basis for the design of three-order non-uniform ration B-spline cures and surfaces for the gear forging die; allowing the contour profile of a formed upsetting impression to be curves when determining the shape of the formed upsetting impression, and allowing points of segments in the curves of the formed upsetting impression to correspond to the feature points K1, K2, K3 and K4 of a gear forging respectively. The complex process of preforming, upsetting and final forging for the gear forgings is simplified, performing of the complex gear forgings can be completed by two procedures, single heating and one tool set, and accordingly cost is effectively lowered and work efficiency is improved.