The invention relates to a method and a system for evaluating a corrosion state of a transformer substation grounding grid. The system comprises a field impact current generation module, a transient current and voltage recording module of all grounding down leads, an impact grounding impedance calculating and impact response curve drawing module, a testing result storing and recording module and a grounding grid state evaluation and analysis module, wherein the field impact current generation module is used for generating impact current; the transient current and voltage recording module of all the grounding down leads is used for collecting the impact current, transient response voltage waveforms of input grounding down leads and impact response voltage waveforms of output grounding down leads in different positions; the impact grounding impedance calculating and impact response curve drawing module is used for processing current and voltage data to obtain the grounding grid impact impedance and an impact response curve of different positions; the grounding grid state evaluation and analysis module is used for evaluating the corrosion state of the transformer substation grounding grid. Compared with the prior art, the method and the system, disclosed by the invention, have the advantages of simplicity, accuracy and capacity of being not limited by a field running condition.