The threedimensional pipe model on Electrical Capacitance Tomography’s(ECT’s) array electrodes was built by COMSOL, the potential cloud was obtained under different array electrode structures, the capacitance values of threedimensional electrodes were solved in the case of empty field and full field respectively, and an expression used to calculate quickly sensitivity filed was deduced according to the characteristics of small pipe in oilair lubrication system. The lateral sensitivity and the sensitivity homogeneity were defined and employed to evaluate ECT′s array electrode characterization of oilair lubrication system, combined with another two traditional assessment index, the sensitivity and the range of measured capacitance values. The results show that the sensitivity of relatively uniform distribution and better detection sensitivity can be obtained by analyzing characteristics of ECT’s array electrode. This research provides a basis for design and manufacture of ECT’s array electrode of the oilair lubrication system.