Abstract: A portable ratio tester for current transformerwas developed.This tester is very suitable for outdoor online testing.It adopts DSP technology.The fundamental current waves' ratio is obtained by soft-synchronous-sampling and FFT,which can greatly decrease the measurement errors caused by harmonic interferences and realize the real-time high measurement precision.Besides the tester system can be used as current transformation for low voltage.Moreover,it can be used by the power station security system to prevent the illegal electricity users.