Abstract: In order to monitor the quality of lubricating oil quickly so as to replace the oil timely, a theoretical analysis method of dielectric constant was used to study the correlation between the iron abrasive grit content, moisture content and total acid value of the lubricating oil and the dielectric constant increment. The results reflect that the dielectric constant would increase with the iron grits content, moisture content, and total acid value of lubricant oil. On this basis, a lubricant dynamic monitoring device was developed. Experimental results show that the device can monitor the quality of lubricating oil of automotive engines in real time, so that the lubricating oil can be replaced timely according to the oil quality once the dielectric constant value of lubricating oil is out of the range from 4.7 to 4.2.