The phase selection mechanism and eutectic growth kinetics of Nb_81.7Si_17.3Hf alloy were investigated by electrostatic levitation technique. The maximum undercooling of this alloy reached 404 K(0.19T_L). By analyzing the cooling curves, its hypercooling limit was derived as 527 K(0.24T_L). A critical undercooling of 194 K was determined for the transition of solidification path. Below this undercooling threshold, (Nb) phase firstly nucleated and grew as primary dendrites, resulting in the enrichment of Si and Hf in the residual melt, which would be beneficial to the formation of the (Nb)+αNb₅Si₃ eutectics. Therefore, (Nb)+αNb₅Si₃ lamellar eutectics formed in interdendritic spaces. With the increase of undercooling, the growth velocity of primary (Nb) dendritic followed a power function, while the eutectic growth velocity increased slowly. The maximum values of (Nb) dendritic reached 89.4 mm∙s^-1. A modified LKT/BCT model was used to calculated the growth velocity of (Nb) dendrites. The results were in good agreement with the experimental values, indicating that the LKT model can be used to describe the rapid dendrite growth behavior of the (Nb) phase in the Nb_81.7Si_17.3Hf alloy melt with a few modifications. Meanwhile, the lamellar spacing of (Nb)+αNb₅Si₃ eutectics notably decreased to 360 nm at 194 K undercooling. Above the critical threshold, the primary (Nb) dendrites disappeared, the whereas (Nb) and Nb₃Si phases nucleated independently in the undercooled liquid and grew as anomalous eutectics. The anomalous eutectic growth velocity increased with undercooling as a power function with a maximum value of 115.9 mm∙s^-1. The interphase spacing of (Nb)+Nb₃Si anomalous eutectics was larger than that of (Nb)+αNb₅Si₃ lamellar eutectics. Owing to the formation of nanosized eutectics and the increased volume fraction of (Nb) phase, the alloy fracture toughness at 194 K attained 21.9 MPa∙m^1/2, which was 3.4 times of that under small undercooling condition.