Abstract: Using sunlight transferred through an energy-transfer fiber for indoor illumination is a new development of solar energy utilization, but there is a technical problem to couple the sunlight into a fiber. A light-focusing device which could couple the sunlight into a energy-transfer fiber was put forward based on the study of plastic fiber characteristics and various light-focusing technology. Based on the traits of the aspherical lens, the sunlight was focused on a very small spot, and then it was transferred into a plastic fiber by an aspherical lens and a selfoc lens. A formula for calculating the optimal length of selfoc lens is presented. The optimal assembly of the aspherical lens and selfoc lens was implemented by the aid of this formula. Therefore, sunlight is coupled into a plastic fiber efficiently. The feasibility of this device is proved theoretically.