Abstract：In order to analyze the impacts of integrator on protective relaying, transient characteristics of air-core current transformer based on Rogowski sensing head with analog and digital integrators are studied by theoretical analysis and simulation, and the main influence factors caused by different integrators are pointed out. It is shown that current transformer’s transient characteristics with analog integrator are improved in low frequency band area comparing with the simple first-order RC integrator, and can meet the requirements of relay protection in theory by choosing suitable parameters. It is also revealed that current transformer with digital integrator and suitable numerical formula presents very excellent frequency characteristics and transient characteristics, and can satisfy the requirements of relay protection for measuring fault transient current.