A spacer for fixing a distance between adjacent lenses of a lens module carries an internal shape reducing the incidence of stray reflected light. The spacer defines a through hole to allow light to pass and a prismatic protrusion is disposed on a wall of the through hole. A first inclined surface of the prismatic protrusion is connected to an end surface of spacer and to a second inclined surface of the prismatic protrusion. The second inclined surface is connected to other end surface of spacer. Acute angle θ1 is formed between the first inclined surface and end surface, and acute angle θ2 is formed between the second inclined surface and other end surface. Relationship between the acute angles are θ1>θ2, and 60°≤θ1+θ2≤80°. A lens module using such a spacer is also provided.