A light source mechanism for an imaging apparatus comprises at least a point light source and a light shield. The point light source located at one end of the light shield is used for providing illumination to the imaging apparatus during a scanning process. The light shield is a hollow pipe structure and further includes a long slot lying along an axial longitudinal direction thereof for allowing lights to leave the light shield. An inner surface of the light shield is made as a diffusion surface for evenly diffusing lights inside the light shield, by which a homogeneous line light source for the scanning process is formed at the long slot of the light shield.