An optical imaging system includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens. The first lens has positive refractive power and a convex image-side surface. The second lens has negative refractive power. The third lens has negative refractive power. The fourth lens has negative refractive power and an inflection point formed on an image-side surface thereof. The fifth lens has positive refractive power and a convex image-side surface.