Taking a certain kind of low-speed diesel engine as an example, the distributions and variations of the temperature, pressure and composition inside the cylinders were output by a simulation software. By introducing the chemical kinetic model of the SO₃ formation, the change rule of the acid dew-point with the crank angle was calculated. The results show that the acid dew-point temperature decreases as the piston go down; the in-cylinder temperature decreases more rapidly than that of the acid dew-point, therefore, the acid low temperature corrosion is more likely to occur around the BDC, and should be taken into consideration the design of the cooling system.