An analysis was conducted on the circulating flow process of coolant in the cooling water jacket of motorcycle single cylinder engines, twin cylinder engines, and four cylinder engines. The analysis of coolant flow velocity and water jacket wall heat transfer coefficient were conducted. Based on the analysis of the circulating flow path of coolant, the optimization of the cooling water jacket structure was studied. The results show that the coolant flow rate on the exhaust side and nose bridge area of a single cylinder engine is relatively low, and there is a zero flow rate area in the nose bridge area. The distribution of coolant flow rate in the two cylinders of a twin cylinder engine is uneven, and the coolant flow rate in the middle area of the connection between the two cylinders is relatively low. There is also an issue of uneven coolant flow rate in each cylinder of a four cylinder engine. By analyzing the flow path of coolant circulation, the flow direction and function of the coolant on the water holes on each cylinder gasket are clarified. Based on the design criteria that require key cooling in high-temperature areas, the layout of the water holes on the cylinder gasket and the local flow area of the water jacket are optimized for three types of engine cooling water jackets. After optimization, the coolant flow rate in the high-temperature areas such as the exhaust side and nose bridge area of the three types of engine cooling water jackets reached the requirement of not less than 1.5 m/s, and the difference in coolant flow rate between each cylinder is reduced. Through engine thermal balance verification, the optimization of the cooling water jacket structure for three types of engines can effectively reduce the temperature of the cylinder head spark plug gasket and reduce the temperature difference between the spark plug gaskets of each cylinder, verifying the effectiveness of the water jacket structure optimization design.