The present disclose relates to a lens. The lens includes a bottom surface; a light input surface depressed from a center of the bottom surface; a light output surface opposite to the light input surface, the light output surface comprising a concave surface located at a center thereof and a convex surface surrounding the concave surface; and total reflective side surfaces. The side surface includes a first and a second side surfaces gradually slanting outwardly along a direction from the bottom surface to the light output surface, and a third side surface and a fourth side surface being perpendicular to the bottom surface. The present disclose also relates to a light source module with the lens.