Transformers neutral grounding accessing small reactance is an effective measure to restrain short circuit current in 500kV power grid. But accessing small reactance will bring changes to the zero-sequence parameters of system. The relay protection, especially the main transformer protection and transmission line protection nearby will be affected and act incorrectly when unsymmetrical earth fault occurs in the transmission line. An equivalent system model based on the Guangdong power grid's actual parameter and autotransformer in 500kV substation is built, the effects on power system's zero-sequence parameters as only one autotransformer neutral point grounded via small reactance is analyzed Meanwhile, a quantitative calculation is accomplished based on the relative variation of zero-sequence components from three aspects: fault position, resistance value and transmission line length. Furthermore, the effects on protective relaying and coordination between the adjacent lines are analyzed according to curves drawn by Matlab. Zero-sequence current protection of autotransformer neutral should be reset to meet sensitivity.