The coupling loss between optical devices is a critical factor affecting the performance of optical interconnect. This paper quantitatively studies the effectiveness of using a dye-doped-epoxy-based self-written waveguide (SWW) to reduce the coupling loss in optical interconnect caused by large mode-field mismatch and lateral offset. We formed SWW between single-mode fiber (SMF) with different mode-field diameters (MFD) and a 5 × 2 µm rectangular channel waveguide-under-test (WUT). For the case between a SMF with a mode-field diameter of 9.4 μm and the WUT, the coupling loss is -11 dB. After forming the SWW, the coupling loss is reduced by 8.34 dB. Using SWW, the lateral tolerance length between a SMF with a mode-field diameter of 4.5 μm and the WUT increases by 2.5 times. Under the above-mentioned situation, the coupling loss falls less than 0.20 dB over ± 2 μm lateral offset range. Our findings offer insights quantitatively for coupling loss reduction and relaxing the lateral tolerance under significant mode-field mismatch conditions.