To cope with the detection difficulty of cross-country fault in non-effectively grounded distribution networks, a cross-country fault detection method is proposed, which is based on the approximate entropy of bus voltages. This method calculates the approximate entropy for bus three phase voltages, and detects whether a cross-country fault occurs during the time window. The cross-country faulted phase can also be detected. The proposed method only uses voltage information, which can not only detect the cross-country fault of the same phase with the primary fault, but also can detect the different phase intermittent fault as well. The time resolution of cross-country faults identification is of millisecond level. The correctness of the criterion is proved by ATP/EMTP typical fault simulation results. The proposed algorithm enriches the single line to ground fault detection techniques.