This paper proposes an algorithm for shadow-removal based on a single outdoor image. The algorithm consists of three steps: first, the study the characteristics of outdoor illumination and an color space to detect the shadow areas in the outdoor scene. Second, by taking detected shadows as background, the paper incororates the matting method to calculate the matting scale factor, with which the penumbra can receive consistent lighting. Finally, shadow pixels are relit by multiplying the corresponding scale factors derived from the matting scale factor. The experimental results demonstrate that the proposed approach can not only detect the self-shadow and cast-shadow, but also produce satisfactory shadow removal results in various outdoor scenes.