Proposes an approach to measure the numerical aperture (NA) of the infinite conjugate objective. It utilizes the total internal reflection (TIR) phenomenon occurring on the air-coverslip interface to generate a reference on the back focal plane (BFP), and the NA is measured based on Abbe sine condition. According to the principle, a home-designed measurement system is developed, containing the optical system, the control system and the whole image-processing software to extract the NA value from experimentally acquired BFP images. The algorithm of the image-processing software utilizes the two-dimensional Fourier correlation analysis and grayscale statistics. The NA values of several objective lenses are measured experimentally. The results indicate that the relative error of the proposed method is no more than 1% , much lower than that of the other method compared in this work. The proposed method features on the simple measurement configuration and fast and automatic measurement of the NA of practical oil-immersion objective lenses. The results show high precision and good repeatability. The approach works for the NA measurement of the infinite conjugate oilimmersion objective lenses.