Micro-distance spectra splitting is a key technology for compact dual-band imaging system in infrared field. Utilizing the Fresnel diffraction theory and angular spectrum theory, the diffractive properties of dual-wavelength and wide angle light passing through stepped phase grating in fractional Talbot distance were theoretically investigated. A typical scheme of imaging structure based on dual-array detection pixels was set up, where two light beams with different wavelength incidented after passing through the stepped phase grating. The numerical simulation shows the splitting results of incident light of 4 μm and 4.5 μm wavelength with -30° to 30° angle, including the distribution of luminous intensity, energy and signal-to-noise ratio on dual-array detection pixels. Our results are beneficial not only for designing the compact dual-band optical imaging structure, but also for building up precision spectral separating component.