Social developments are bringing about changes in how we communicate and represent ideas. With the tendency of globalisation, marine ecosystems are facing threats such as overfishing, pollution, and climate change, and marine ecological protection has become a focus of attention for all countries. This paper adopts Kress and van Leeuwen’s framework of visual grammar analysis to compare and analyse the use of symbols in Chinese and Western marine ecological protection posters from the perspectives of representational meaning, interactive meaning, and compositional meaning. Qualitative and quantitative analyses of 100 Chinese posters and 100 Western posters reveal similarities and differences in symbol selection, cultural context, and compositional approach. Results show that Chinese posters tend to integrate traditional cultural elements and natural symbols to convey the concept of environmental protection in a harmonious and warm way, such as the use of ink painting style and paper-cutting art to represent marine organisms, while Western posters focus on directly displaying the hazards of marine pollution by means of strong visual contrasts and modern design elements to stimulate the audience’s sense of urgency and responsibility, such as the display of marine animals trapped by plastic rubbish. This research not only deepens the understanding of the theory of visual grammar but also provides valuable references for the design of future environmental posters, emphasising the importance of using visual symbols and communication strategies in different cultural contexts in order to convey environmental messages more effectively and to promote the development of global marine conservation.