Wide attention has been drawn to the application of space networks (SN) in recent years. Compared with terrestrial networks, SN have a set of unique characteristics, e.g., a long propagation delay, time-varying channel quality, and dynamic link connection, which causes the inapplicability of mature terrestrial networking technologies. Therefore, the focus of SN application is to innovate and break through a series of key networking technologies and protocols. Network emulation is critical for evaluation and verification in the research of networking technologies and protocols. This paper concentrates on designing an emulation platform which provides a realistic, flexible, and extensible experimental environment for the SN. The architecture of platform includes four components, namely the logical plane, control plane, data plane and measurement plane. Container technology is adopted to flexibly symbolize network nodes. The emulation method of dynamic connection relationships and time-varying link characteristics between pairwise nodes are considered so as to realize the real space environment and arbitrary topology. Furthermore, an extensible structure is described to emulate large-scale scenarios and access external emulation resources. Finally, experiment results show that the proposed platform is capable of emulating diverse, dynamic, and complex SN scenarios with high precision.