We present a method for designing a transmissive-reflective combined optical system to generate a focused ring-shaped laser beam. The design aims to achieve a freely adjustable radius for the focused ring-shaped laser beam and ensure uniform beam intensity even after defocusing. Based on the principle of equal energy splitting, the transmissive system establishes mapping functions for the input and output light projection height. It optimizes the lens parameters to shape the incident Gaussian light into a flat-topped circular shape, thus achieving uniformity of beam intensity. On the other hand, the reflective system uses the adjustable diameter range of the focal plane ring-shaped light and working distance parameters. By applying the principle of geometric ray tracing, it calculates the parameters of the conical reflecting mirror, parabolic cylindrical mirror, and dynamic mirror, then the flat-topped circular light is transformed into a ring-shaped light. The experimental results show that when the half-apex angle of the dynamic mirror is 16°, the designed system can achieve a freely adjustable radius for the focused ring-shaped laser beam from 15 mm to 30 mm with a size error not more than 0.05 mm, and the intensity uniformity after defocusing reaches 84%. The design method can achieve both uniformity of intensity and freedom of size adjustment without replacing the system lens. It has good operability and yields higher precision and efficiency in the processing of ring-shaped light.