In view of light-emitting diode (LED) light source’s characteristics, according to the étendue conservation theory in non-imaging optics, the improved reflection equations for obtaining uniform illumination are derived and the uniform illumination in specific target plane is achieved. Based on the equations, an illumination system of LED-base stereomicroscope is designed. Using software Tracepro, the ray-tracing simulations show that, in the scope of Φ90 mm, the uniformity of illumination reaches 90.6%, and the efficiency can reach 99.6% without considering the loss. This scheme uses only one single reflector to achieve uniform illumination and provides an effective way for realizing system’s miniaturization and simplification.