The coastal areas are the most economically developed, and they need to build many infrastructures such as airports, ports, and subways. However, the complex geological conditions in the coastal areas pose risk issues to the construction. In response to the characteristics of the high risk of subway shield tunneling, we proposed a new method to evaluate the safety of subway shield tunneling by using the entropy weight method and the matter-element theory. In addition, we built a safety evaluation index system with 52 evaluation indexes, including workers, machinery, materials, technology, environment, etc. Accordingly, using the entropy value method, we calculated the weight of each index and built the classical domain with the matter-element theory. Then, combining the joint domain with the evaluation of the main factors, we calculated the risk correlation degree. Moreover, we established the safety assessment model. Taking the coastal tunnel between the Hujing Station and Wanshou Station of Fuzhou metro line 6 as an example, we validated the evaluation model and assessed the risk of the coastal tunnel. The result demonstrates that the total construction safety risk level is low. But the environmental factors are very crucial; the highest is the embankment collapse C523, and its proportion is 8.14%. It is necessary to measure deformation and reduce the environmental risks of construction. The case study can provide a very valid reference for some similar coastal projects.