In recent years, with the rapid development of marine transportation and port logistics and international trade, the traffic density and flow of ships at sea are increasing, which brings many challenges to the safety of maritime navigation and marine ecological environment. The application of Automatic Identification System (AIS) technology provides a good data source for maritime traffic management and marine spatial planning. Based on the AIS data of the Bohai Sea in 2018 with data quality control and sampling processing, this study analyzed the traffic characteristics of the Bohai Sea. By means of mathematical statistics and Geographic Information System (GIS) spatial analysis, the spatial distribution characteristics such as ship position and density, ship speed and the temporal characteristics of daily, monthly, and seasonal changes of ship flow in the Bohai Sea were analyzed in order to reveal the spatial-temporal variations of ship activities in the Bohai Sea. The influencing factors of the spatio-temporal distribution of maritime ship activities were also discussed. The results show that: 1) Fishing ships were mostly located in the offshore area, which were agglomerated. Commercial ships were mostly gathered in the common area, which were distributed in belts. The number of cargo ships accounted for 78.58% of commercial ships and they were mainly medium-sized cargo ships, and passenger ships and oil tankers were mainly small and medium-sized ships. 2) Speed of the ships was mainly medium and low, especially fishery ships. The ships with higher speed were mainly large and medium-sized freighters and passenger ships. 3) The activity time and seasonal change characteristics of fishery ships were significant, and activities were more frequent after the end of the fishing moratorium and during the daytime. Compared with fishery ships, the activities of commercial ships in the Bohai Sea were less affected by seasonal changes and day and night alternation. It is found that the Bohai Sea traffic flow density was high, ships met frequently, and the risk of ship collision and major pollution accidents increased through the study period. There is a large conflict between the east-west and south-north marine traffic and fishing ships in the utilization of maritime space, and more attention needs to be paid to this conflict. Natural environmental and resource conditions, coastal socioeconomic development and port construction, regulation of the sea, and other factors affect the spatio-temporal distribution of marine ships.