The fundamental mode cut-off and single-mode conditions of asymmetric ridge waveguide under different ridge heights, ridge widths and the short side widths of core plate are systematically studied in semi-vector finite-difference beam propagation method (FD-BPM) with organic polymer asymmetric ridge waveguide as the object. The result shows that both fundamental mode cut-off area and single-mode area increase with reduction of ridge height and ridge width while single-mode area decreases with reduction of short side width of core plate at a certain core height. The structure is optimized as core height h≥1.5 μm, ridge height (H-h)≤0.4 μm, ridge width 4 μm≤w≤8 μm, short side width s=2 μm. The research shows that the single-mode area of asymmetric ridge waveguide meets requirements of practical application.