As a strategic national asset ensuring the security of China’s energy resources, the summer extreme heat environmental condition and complex air flow composition of the ocean drilling vessels have become one of the key challenges in the design and construction. By establishing the balance formula of air volume inside a galley, this study sorts and analyzes the airflow parameters such as air conditioner air supply, air intake outside and inside the cabin and air mechanical exhaust. Based on the cooling capacity calculation method for fresh air conditioners specified in ISO 9943:2009, and incorporating personnel comfort requirements, this study proposes a solution for adjusting air conditioning cooling capacity parameters under extreme heat conditions exceeding standard environmental design specifications. Using an ocean drilling vessel as a case study, calculations were performed to develop recommended cooling capacity parameters for such extreme conditions on this vessel. This provides a reference for engineers to clarify air flow composition in complex cabins and to select appropriate cooling equipment under extreme heat environments.