A simple thin lens optical system fully equal to selfoc lens is got by using the method of matrix optics. This equivalent optical system is composed of a positive thin lens and two same optical intervals. Under the condition of paraxial approximation, the system can substitute a selfoc lens to simplify the optical analysis of an optoelectronic system containing complex selfoc lenses. Imaging properties of a selfoc lens at various lengths are studied and the object-image relation of a selfoc lens at various lengths is presented by using this equivalent optical system. Based on this, the application of the equivalent optical system, such as fiber collimation, optical system of photocopy machine, and fiber optic coupler, is discussed.