According to the characteristics of the reflective optical microscope lighting system, an improved Köhler illumination system for the full-field optical coherence tomography system (FFOCT) was designed to realize the illumination of biological samples and living biological tissues. The illumination system differs from the conventional Köhler illumination system. The filament of the halogen lamp is imaged on the back focal plane of the microscope objective, then parallel light is incident on the sample plane. The improved Köhler illumination system uses a halogen lamp as the light source and is divided into two parts: The condenser front and rear groups. The front condenser group uses two double-glued structures, and the rear group uses a double-coupled lens. The optical design software Zemax was used to optimize the design, and the illumination analysis software Tracepro was used to trace the ray and simulate the imaging of the light source in the front focal plane of the microscope objective. The entire improved Köhler illumination optical path has a total length of 594 mm, the diaphragm is 122 mm from the front group of the condenser, 99 mm from the rear group, and the working distance is 292 mm; the luminous efficiency of the receiving surface is as high as 60.38%, and the edge of the light spot is smooth and clear. The illumination system makes full use of the optical power emitted by the light source and facilitates the placement of a device such as a splitting prism between the condenser and the microscope objective, which satisfies the requirement of the entire machine well. © 2019 COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.