We report a design method based on an annularly stitched aspherical surface for highly collimated optical systems with extended light sources. The annularly stitched aspherical surface is constructed as a circular central zone and one or more annular zones, which are rotationally symmetric and provide flexibility to satisfy the requirements of a practical design. An extended LED light source can be considered, and a backward ray-tracing process is used to construct the initial lens model. The specific optimization has been added step by step to modify the stitched surface to redistribute the light rays emitted from the extended source. The smoothness of the stitched surface is considered in the design and optimization process to guarantee the processability of the system. A prototype has been fabricated and tested, which demonstrates the validity of this method.