To solve the existing problems in the single-phase ground fault line selection approach for small-current system such as lower accuracy and poor reliability, a single-phase ground fault line selection approach for small-current system is proposed based on unscented Kalman filter (UKF) and sequential probability ratio test (SPRT). First, a mathematical model of fault line is established based on UKF. Then, UKF is used to track and predict the characteristic information of system fault. The resulting residuals between the observed value of new information and the actual measured value are used to calculate the likelihood ratio using the sequential probability ratio method. Moreover, a fault criterion is constructed, which can be used to quickly select the fault line. Results of a large number of simulations using ATP-EMTP and Matlab show that the proposed method has higher accuracy and reliability.