A new method based on calculating approximate entropy of transient zero-module current is presented in order to locate the single phase grounding fault in non-solidly earthed network. The approximate entropies of transient zero module current at both ends of healthy section are approximately equal each other， so the ratio is close to one. On the contrary， the approximate entropies at both ends of fault section are different， and the ratio is less than that of healthy section. According to this characteristic， the earth fault location can be determined. Accurately synchronous sampling is not a prerequisite in this method. Simulation and site test results prove the accuracy and feasibility of the new method.