The invention discloses an illuminating system for focusing and leading sunlight into a building north window. A light condensing module is a bent cylinder structure opened at two ends; multiple convex lenses are uniformly arranged in the upper end opening of the light condensing module, and main light shafts of the multiple convex lenses are intersected in one point A in the light condensing module; a light condensing ball is a glass ball body with an elliptic opening, and is detachably arranged in the light condensing module through a support piece; a projection reflecting cover covers the light condensing ball; a strip-shaped opening is formed in the projection reflecting cover; one end of a light guide pipe is communicated with the lower end opening of the light condensing module; a reflecting module and a wall are vertically sleeved at the outer periphery of the window; a reflecting surface of the reflecting module is a step-shaped structure; and the other end of the light guide pipe extends and penetrate to at least one reflecting monomer. The illuminating system can provide natural light rays indoors to increase the brightness around the building north window without shielding the window scenery and occupying indoor space.