The invention discloses a small reactance value choose method during ultrahigh voltage autotransformer neutral point grounding in a small reactance mode. According to the principle that during neutral point grounding through an electric reactor, the ratio of the reactance value Xn of the neutral point to the zero-sequence reactance value X0 of a transformer is less than or equal to 1/3, according to the symmetrical component analytical method and specific boundary conditions of a dissymmetric short-circuit, by means of a BPA short-circuit current calculation procedure, a high-voltage side bus and a low-voltage side bus of a ultrahigh voltage autotransformer in a transformer substation are subjected to one-phase grounded short-circuit current calculation to choose a suitable reactance value, during the neutral point grounding through the electric reactor, the ratio of the reactance value Xn of the neutral point to the zero-sequence reactance value X0 of the transformer is less than or equal to 1/3, namely, Xn/X0<=1/3, the guarantee is provided for the reliability of monitoring the normal operation during the autotransformer neutral point grounding in a small reactance mode, and the wide practical significance is achieved.