In terms of Helmholtz equation and finitedifference timedomain method, a numerical approach for analyzing the directional coupling characteristics based on two SU8 filled slot waveguides was proposed. The mode eigen equation of the coupling slot waveguides was derived and solved, and mode optical field distribution and effective refractive index were achieved. By using the effective refractive indices of the highestorder even symmetric mode and the highestorder odd symmetric modes, the coupling length under TE and TM polarizations was calculated. Numerical simulation results show that when the coupling gap is less than 800 nm, the coupling length is less than 100 μm. The wavelength dispersion characteristics of the used materials were measured with an ellipsoidmeter, and then the wavelength dispersion of coupling length and mode loss of the coupling slot waveguides were investigated. The analytical results reveal that the coupling length decreases as the wavelength increases under both polarizations, and under the same coupling gap, the coupling length under TM polarization is larger than that under TE polarization. The coupling lengths under TM and TE polarizations are 28.2 and 26.2 μm, respectively, when wavelength is 1 550 nm and coupling gap is 0.5 μm. In addition, the mode loss also decreases as wavelength increases, and the mode loss under TE polarization is larger than that under TM polarization. The loss coefficients under TE and TM polarizations are 5.65 and 3.16 dB/cm, respectively, when the wavelength is 1 550 nm.