Maritime transport is a major mode of transportation. Over 80% of international freight is carried by this mode. A port is a hub of ships and freight in maritime transport. Because of growing environmental concerns, how to effectively monitor, control, and improve ship emissions in a port has become a challenge for port administrations. This study combines automatic identification systems (AIS), ship emission estimation model (SEEM), geographic information system (GIS) mapping, and a scenario simulation technique to create a ship emission scenario simulation model (SESSM) for mapping and assessing current ship emissions alongside various “what-if” improvement options in a port area. A case study of the Port of Keelung in Taiwan is used to illustrate and verify the proposed model. In this case, the distribution and density of ship carbon emissions are mapped, with the ship berthing status being identified as the primary source of ship emissions. Meanwhile, nine “what-if” scenarios based on various combinations of speed policies and shore power supplies are simulated and analyzed. The results show that the proposed scenario simulation model is an effective tool to assess various “what-if” emission improvement options and to identify key factors for emission reduction. The effect of shore power supply on carbon emission reduction is significantly greater than speed policies. If investment costs are an issue, a balanced emission improvement option is suggested by combining a new speed policy and 50% shore power supply.