Aiming at the adverse effect of high reflection and high curvature of metal sphere on surface micro-defect detection, a detection method based on spatial omni-directional light source and image fusion is proposed. The image information is obtained by scanning the spherical surface through different illumination schemes provided by the spatial omni-directional light source system. Image fusion is used to accumulate the difference between micro defects and background in the effective detection area, enlarge the difference between micro defects and background, and improve the ability of micro defect detection. The experimental results show that the detection device based on the proposed method can detect the micro defects at different positions on the surface of bearing steel ball with a diameter of 20 mm and a precision of G16 significantly, and solves the problem of blind area caused by high reflection of metal sphere, and has good detection ability for various micro defects on the surface of bearing steel ball.