With the rapid advancement of autonomous vessel technology, future port channels will witness mixed navigation involving both autonomous and conventional ships. A key challenge lies in ensuring navigational safety while enabling effective coordination between autonomous and traditional vessels during port entry and exit, thereby maximizing channel throughput. This paper proposes a formation strategy for autonomous vessels tailored to mixed traffic scenarios in ports and develops a robust macroscopic modeling framework to assess the impact of such strategies on channel capacity. Two formation strategies are introduced: cooperative formation and random formation. A comparative analysis is conducted under varying traffic demands and autonomous ship penetration rates. Experimental results show that at medium penetration rates, the impact of autonomous formations on channel capacity is minimal; however, at high penetration rates, the impact becomes significant. The cooperative formation strategy outperforms the random one in terms of capacity enhancement. The proposed formation strategies for autonomous ships can effectively enhance channel capacity in mixed navigation scenarios, providing theoretical support and technical guidance for future optimization of port and channel traffic management.