A wider bio-optical waveguide is always desirable for large-field illumination of an optical waveguide microscopy. However, a wider waveguide leads to non-uniform illumination due to non-uniform evanescent field excited by multi-mode interference (MMI) patterns of waveguides. We introduce a seven-layer model of bio-waveguide for optical waveguide microscopy to analyze the waveguide-cell system. We also develop an averaging strategy of MMI pattern images by moving the coupling position to generate uniform illumination inside a wide bio-optical waveguide. Then, we systematically investigate the averaging strategy of MMI pattern images due to wide waveguide, at the wavelengths of 488, 532, and 646 nm. Finally, we demonstrate a 25% uniform improvement by the averaging strategy inside a wide bio-optical waveguide, compared with the conventional strategy.