With the development of maritime transportation, oil spill accidents occur frequently. In this paper, a scientific and reasonable assessment of ship oil spills, offshore oil platform oil spills, and subsea pipeline oil spills is carried out, and a risk assessment method of an offshore oil spill based on an n-value neutrosophic set is proposed. First, the oil spill risk evaluation index systems of these three risk sources are constructed, respectively, and the entropy weight method is used to calculate the weight. Second, we establish a risk assessment model under the n-value neutrosophic environment. Furthermore, we use the n-value neutrosophic-weighted arithmetic average (NVNWAA) to calculate the risk levels of the following three risk sources: ship oil spills, offshore oil platform spills, and submarine pipeline spills. Finally, according to the results of the risk assessment, the countermeasures to strengthen the preventive measures of oil spill accidents are put forward.