In the study of the interaction between the ultra intense laser and material, it is often focused on the needed quality light source to diagnose the dynamic material structure. In order to obtain a high-quality X-ray source featured with high luminance, quasi-monchromaticity and good contrast, the changes in the structure of matters and materials are often investigated to enhance the absorbability of the ultra intense laser energy and improve the conversion efficiency from the laser radiation to the X-ray. Based on a porous structure principle, the velvet Cu targets were developed with a diameter of 200 nm and the density ratio of 70% solids of Cu. Experiments had been carried out on XG-Ⅲ laser facility in Laser Fusion Research Center, China Academy of Engineering Physics. The laser intensities in these experiments ranged from 4.3 J to 6 J on the target surface. A single-photon- counting X-ray CCD was used to measure Kα spectrum of the X-ray source. The X-ray yield was counted to achieve Kα peak photons of 3.6×108 photons·sr-1·s-1 from femtosecond irradiated velvet Cu target. The Kα X-ray conversion efficiency (CE) reaches the maximum value 0.008 68%. Compared with the average CE of the pressed foil Cu target, that of velvet Cu increased 1.2 times. The experimental data showed that the velvet structure can effectively enhance the energy absorption of femtosecond laser and improve the conversion efficiency from the ultra-intense laser to the hot electron and X-ray.