The diesel engine oils diluted by diesel oil were detected using FT-IR infrared spectrometer,and the correlation between the diesel oil content in lubricating oil of the diesel engine and its characteristic infrared absorbance were researched through the Minitab software. The results show that the diesel oil content in lubricating oil can be detected quickly and effectively by the infrared spectroscopy,and a good linear relationship is found between the diesel content in on line lubricating oil of the diesel engine and the characteristic infrared absorbance,the correlation factor is 0.05.