Aiming at the problem of defect defection of the small holes at inner surface with the diameter below 3 mm, a new defection method combining the micro optics and multi-focus image fusion technology was proposed. Firstly, in the different light illumination, the inner surface images of the hole were collected along the axial direction of the hole at oblique incidence. Secondly, the Region Of Interest (ROI) was extracted from the collected hole images based on the mask template obtained by foreground illumination, which was registered through Speeded Up Robust Feature (SURF) algorithm. In the following step, the ROI images were fused through the multi-focus fusion method based on definition of region and wavelet transform. Finally, the guide hole of the spinneret plate with the diameter of 2 mm was used as the experimental object, and the corrosion spots extracted by the segmentation algorithm based on threshold were used for detection and analysis. The research results indicate that the proposed method is certain feasible, which avoids the low efficiency of artificial detection, and breaks the limitation of the traditional defection methods for small holes with diameter below 3 mm.