The measuring principle of absolute reflectance factors was discussed, and a specific radiometer to measure both the incident irradiance and reflected radiance and a reflectometer to measure the absolute reflectance factor were designed. On the basis of the measurements of the incident irradiance and the reflected radiance, the formula for calculating the absolute reflectance of a diffuse panel was deduced, then in consideration of the measuring accuracy depended on the aperture area, a new method was proposed to measure the aperture area to achieve an exact solid angle. Finally, the design of reflectometer and the fabrication of related devices were introduced. The reflectometer was used to measure the absolute reflectance factors of a standard diffuse panel at visible and near-infrared wavelengths(633-960 nm) and results show that the uncertainty obtained by this system is 0.19％. These results demonstrate that the measuring method is simple, and can satisfy the requirements of reflecting factor measurement for the accuracy.