Based on a typical case that the backup protection of high voltage side tripped earlier than the backup protection of low voltage side when two-phase short circuit fault took place at the transformer low voltage side, this paper analyzes the characteristics of the currents of high voltage side and low voltage side when two-phase short circuit fault takes place at the transformer low voltage side. Considering the situations of other kinds of faults at the transformer low voltage side, this paper finds out the difference of sensibility of backup protections between the high voltage side and low voltage side. The paper finds that the provision DL/T584-2007 3~110 kV operation setting regulations of relay protection equipments of power network in the standards of the power industry is inappropriate. Hereby, this paper gives an advice on the setting of transformer’s backup protection: the setting sensibility of backup protections of high voltage side must assort with that of the low voltage side, and the fit coefficient must be not less than 1.15.This proposal effectively promotes the reliability of transformer’s backup protection of high voltage side and low voltage side.