Signal injection based fault line selection protection identifies the fault line and locates the fault point by finding the current loop of injected signal. It has been widely used in actual power systems. But operational experience of the protection shows that it cannot work properly when the fault resistance is large. The cause of the phenomenon is carefully analyzed in the paper and the characteristic formula used to calculate the critical fault resistance is deduced. Moreover， some modification methods are proposed to increase the critical fault resistance， such as lowering the frequency， adopting the phase information of injection signal， and introducing double frequency.