Based on the characteristics of capacitive sensor in oilair lubrication ECT system of small pipe diameter,twodimensional mathematical model of capacitance sensor in ECT system was built.The effects of relative dielectric constant on the edge capacitance were analyzed,the edge capacitance influence coefficient was defined,and the relation between the edge capacitance influence coefficient and the pipe radius was studied.The results show that the ratio of the tube wall thickness,the oil film thickness and the gas thickness has significant effects on the edge capacitance value,and the edge capacitance can be effectively reduced by reducing oil film thickness and tube wall thickness.The edge capacitance influence coefficient in small pipe diameter is more than 05,and the edge capacitance influence coefficient is decreased with the pipe diameter increasing.The edge capacitance of the sensor has a great effect on the spatial graphic imaging error of ECT in the oilair lubricating system.