The invention provides a detecting system and a method for the lithology of an ultrasmall-caliber drilling hole. The detecting system comprises a drilling hole data acquisition module; the drilling hole data acquisition module comprises a metal shell probe, an image pickup unit and a natural gamma ray unit, wherein the image pickup unit and the natural gamma ray unit are arranged in the metal shell probe; the image pickup unit is arranged at the top part of the metal shell probe and comprises a vertical camera or a side camera or combination of the vertical camera and the side camera, wherein the vertical camera is arranged at the top part of the metal shell probe; and the side camera is arranged at the side wall of the metal shell probe. The invention is used for detecting the lithology and the layering thickness of the ultrasmall-caliber drilling hole, and can distinguish the lithologies and the layering thicknesses of the main lithology of the sedimentary formation such as grit, medium sandstone, fine sandstone, siltstone, yakatagite, mudstone, aluminium-soil mudstone, carbon mudstone, coal seam, limestone and igneous rock and the like mainly for the common sedimentary rock formation.