One embodiment of the invention discloses a wide-angle high pixel density image pick-up optical system and a lens applying the same. From an object plane to an image plane along an optical axis of the system, the system comprises at least a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens, a seventh lens, an eighth lens and a ninth lens which are successively arranged; the optical system satisfies following conditions: (1) -0.3<f / f1<-0.12; (2)-0.5<f / f2<-0.1; (3)0.01<f / f3<0.03; (4)-0.1<f / f56<0; (5)0.05<f / f89<0.45, wherein, f represents the focal distance of the whole optical system, f1 represents the focal distance of the first lens, f2 represents the focal distance of the second lens, f3 represents the focal distance of the third lens, f56 represents the combined focal distance of the fifth lens and the sixth lens, f89 represents the combined focal distance of the eighth lens and the ninth lens. On the other hand, the embodiment also provides a lens. According to the embodiment of the invention, the optical system or the lens is constituted of nine lenses and has a simple structure; in addition, the optical system or the lens has excellent performances such as ultra-wide angle, large aperture, low color aberration, and high pixel density of imaging, and therefore images with high pixel density are obtained.