The invention discloses an optical imaging system, which sequentially comprises a first lens group and a second lens group from an object side to an image side. The first lens group at least comprises a first lens having a negative focal power and a second lens having a focal power. The second lens group at least comprises a third lens having a positive focal power, a fourth lens having a focal power and a fifth lens having a focal power. The image side surface of the third lens is a convex surface. The object side surface of the fifth lens is a convex surface. The effective semi-diameter DT32 of the image side surface of the third lens and the focal length f3 of the third lens satisfy the relationship of 0