The invention discloses an optical photographing lens assembly including, in order from an object side to an image side, a first lens element, a second lens element, a third lens element and a fourth lens element. The first lens element with negative refractive power has an image-side surface being concave in a paraxial region thereof. The second lens element with positive refractive power has an image-side surface being convex in a paraxial region thereof. The third lens element with positive refractive power has an image-side surface being convex in a paraxial region thereof. The fourth lens element has negative refractive power. By means of the first lens element with the negative refractive power, it is beneficial to enlarge a system field angle. In addition, the system main positive refractive power are arranged in the second and third lens elements, and the fourth lens element with the negative refractive power is matched, a back focal length can be suppressed, and the lens overall length is prevented to be overlong. The image-side surface of the first lens element being concave and the image-side surface of the second lens element being convex are satisfied, thus the image quality is improved. The invention also discloses an imaging unit formed by the optical photographing lens and an electronic device.