Air cooler system is aimed at increasing the density of pressurized air to reduce the work cycle of the average cylinder temperature and exhaust temperature, to prevent excessive thermal load on components, effectively inhibit the NOx emissions. Daihastu 6PSHdM-26H diesel engine, which is a type of 4-stroke medium-speed turbocharged marine diesel engine, is widely used in ocean-going ships. Considering its working principle and structural properties, this paper describes some roles and running influent factors of air cooler, establishes its physical and mathematical models in order to set up the numerical calculation method, gets the relative parameters of deterioration of cooling capacity in an air-cooler. And then points out that the influence of deteriorated heat transfer in an air-cooler for the effective fuel oil consumption of marine diesel engine is dual , that is, the increasing of air flow resistance and dirt as well as decreasing of cool capacity, the effective fuel oil consumption is rising. On the basis of the numerical calculation of air cooler, these results not only provides some theory support to solve deteriorated of operational performance for heavy-duty 4-stroke marine diesel engine, but also decrease the dependence degree for a marine engineer, then make it possible to scientific manage modern big-scale automatic engine room and improve the reliability of marine propulsion power plant.