Abstract: Aiming at how to accurately determine the short circuit fault in power system in order to improve the reliability and sensitivity of the relay protection of power system, a method which used short-circuit current in a non-periodic component as a criterion in short-circuit fault was proposed. Using Matlab to extract short-circuit current of the non-periodic component,was then applied the features of the non-periodic component in the short circuit current to the relay protection of power system. The test run results showed that the directional differential protection of non-periodic components was not affected by the influence of line to ground capacitance. Either as a sole major protection, or as a part of the combined protections, it can always accurately identify the short fault of electric system. However, non-periodic components are transient signals with short existence, therefore a signal maintainer is needed during operation.