Transient characteristics of digital closed-loop fiber optic current sensors (FOCSs) include the frequency response and step response which are important indexs in the industrial areas such as power system relay protection and fault recording. In engineering applications, due to the complexity of the test system and the poor accuracy of a small current signal, it is not conducive to evaluating the transient characteristics of an FOCS in a traditional way. According to the closed-loop control mechanism of a digital closed-loop optical fiber current sensor, a new method of adding phase modulation excitation to the feedback loop of a digital closed-loop FOCS was researched. First, the phase modulation principle of the digital closed-loop of FOCS was analysed, and a mathematical model was established to calculate and verify the equivalence of the phase modulation excitation; Second, based on the closed-loop control algorithm of the FOCS, the frequency response and step response modulation excitation generation method were proposed; Finally, use the existing digital closed-loop FOCS product platform was used to complete the simulation and test of frequency response and step response. The results show that the maximum error between simulation data and test data is 0.12%. The modulation excitation method can realize different current simulation tests of frequency response and step response on the existing hardware platform without building a complex test system.