Fiber-optic current differential relays should be provided with a fiber-optic digital synchronous interface when directly connected. The relays should be provided with a 64 kbit/s interface or E1 interface when transmitting data through multiplexer（PCM exchangers or the E1 multiplexer）. The two kinds of synchronous interface in conformity with ITU-T G.703 are in general realized in the fiber-optic spire. Several schemes of implementing fiber-optic digital synchronous interface and fiber-optic spire are discussed. The schemes are compared with respect to both the performance and complexity of implementation and a reliable and flexible scheme is obtained， which not only satisfies the requirement on synchronous performance completely， but also brings the software and hardware platform of the fiber-optic digital synchronous interface and the fiber-optic spire into accord. Furthermore， the fiber-optic spire is seamlessly linked with the fiber-optic current differential relay when connected with the multiplex channel.