<p>The space terahertz information network (STIN) leverages the terahertz (THz) frequency band for data transmission, offering advantages of large capacity and high-speed communication. However, existing research commonly overlooks the profound impact of THz-specific characteristics on the performance of the satellite network’s medium access control (MAC) layer and lacks adaptive MAC mechanisms that can fully match THz’s high-speed transmission capabilities. To address this issue, this paper proposes an adaptive MAC scheme customized for STIN. This scheme integrates information from the physical, link and network layers, dynamically adjusting the THz beamwidth and timeslot allocation during each transmission cycle, and selecting the optimal handshake procedure and the MAC access mechanism based on varying traffic loads and channel conditions. The main innovations of this paper are as follows. First, an STIN MAC performance analysis framework combining queuing theory and geometric probability models is established, systematically revealing the constraints imposed by THz characteristics on MAC performance. Second, a multi-parameter cooperative optimization MAC mechanism is proposed, which can significantly improve throughput and link resource utilization in complex dynamic environments. Using terahertz communication between GEO satellites and LEO constellations as a typical scenario, simulations are conducted, and the results show that the proposed scheme achieves up to 27% throughput gain under link fluctuation conditions compared with traditional MAC mechanisms, fully verifying its engineering application potential.</p>