An optical imaging system includes, in order from an object side to an image side, a first lens, a second lens, a third lens, a fourth lens, a fifth lens and a sixth lens. The first lens element with positive refractive power has a convex object-side surface. The second lens, the third lens and the fourth lens have refractive power, and both of the surfaces are aspheric. The fifth lens has negative refractive power and has a concave image-side surface, and two surfaces of the fifth lens are aspheric surfaces. At least one surface of the fourth lens has a point of inflexion. When specific conditions are met, the optical imaging system can have higher light receiving capability and better optical path adjusting capabilities to improve imaging quality.