Two detection systems of fiber current sensors are described and compared in this paper. The analyses indicates that the double way detection system has higher level of SNB and stronger ability of suppressing distrubances than the single way detection system. It is not only used for measuring AC hut also DC and is considered to be the better detection system of fiber current sensors. The ^single way detection system has advanteges of simple structure, convenient adjustment and no special requirement for photodetector and amplifier. It is also useful in other fiber sensors such as fiber voltage meter.