Abstract: In large numerical aperture (NA) spherical surface shape detection, the defocusing errors introduced by the measured sphere deviating from the ideal confocal position will seriously affect the detection accuracy. In order to accurately extract and effectively correct defocusing errors and improve the efficiency of adjusting the final inspection position of components, a defocusing error correction method based on the error extraction algorithm assisted installation and adjustment was designed. The defocusing error extraction algorithm was used to quantitatively extract the defocusing amount and determine the direction of adjustment, so that the large NA spherical surface could be adjusted to zero fringe position quickly and accurately, and the defocusing errors could be greatly reduced. After auxiliary installation and adjustment, Zernike wavefront fitting algorithm was used to eliminate the defocusing term and the spherical difference term introduced by the large NA spherical surface for fine adjustment. The experimental results show that the defocusing extraction errors is within 3% by using the proposed method to correct large NA spherical surface. This method increases the range of defocusing errors extraction and compensation for components, and can extend the upper limit of defocusing amount to ±25 μm, which ultimately achieves the sub-nanometer correction accuracy.