The work aims to study the effects of TD treatment technology on the forming thickness, morphology and surface hardness of the inner wall of the slender hole, and the micro-morphology and formation reasons of the coating formed on the inner surface of the hole by thermal diffusion carbide coating treatment technology under the conditions of different hole diameters and hole lengths. The most widely used vanadium carbide coating formula was applied to coat the sample, and the corresponding vanadium carbide coatings were obtained on the inner wall surface of the sample. The phase composition of the inner surface after treatment was analyzed by X-ray powder diffraction experiment (XRPD). The thickness of the formed coating was measured with an optical microscope. The microstructure of the surface coating was analyzed with a scanning electron microscope (SEM). The bond strength between the coating and the substrate was measured with a scratch tester. The surface hardness was measured with a microhardness tester. A layer of vanadium carbide coating was successfully found on the inner wall surface of holes with different hole diameters. The thinnest part was 3.06 μm of 3 mm hole sample. The thickest part was 5.29 μm of 6 mm hole sample. In the hardness test experiment, compared with the raw materials without coating treatment, the hardness of the inner wall of the hole after coating was strengthened. The strengthening effect of 6 mm hole was the best. The maximum surface hardness reached 1 767.8HV2/15. The thickness of inner surface coating after TD treatment increases with the increase of hole diameter. At the same time, the thickness and surface hardness of the coating are positively correlated with the hole diameter. With the change of hole length, it will decrease first and then increase. The vanadium carbide coating formed can effectively improve the surface hardness of the material.