Flexible transfer converter (FTC) positioned at the grid interface can enable fully autonomous control of microgrids through automatic synchronization control (ASC), automatic frequency control (AFC) and automatic voltage control (AVC) and so on. Back-to-back (BTB) current source converter (CSC) shows potential advantages working as FTC, like smaller footprint and lower cost because of the elimination of step-up transformers. Different from the BTB voltage source converter, BTB CSC’s dc voltage is not required to keep constant in operation. Considering this characteristic, the dc-link current minimization by modulation ratio maximization can be selected as the strategy to optimize the system efficiency. Its internal mechanism is analyzed in detail by comparing the quantitative relationship between degrees of freedom and controllable variables, indicating that the modulation ratio of CSC on one side should be maximized. On this basis, the control architecture of BTB CSC-based FTC is established, which includes the top layer control consisting of ASC, AFC and AVC as well as the bottom layer control for tracking power references. Finally, the effectiveness of current-source FTC with the proposed control strategy is verified by simulation results.