By employing Marcatili's principle, the two-dimensional spatial four-waveguide system is decomposed as two perpendicular five-layer slab waveguides independently in the cross-section of the waveguides. The analytical model that can be interpreted appropriately by variable seperation method is proposed. The first order correction for deviations of the model and practical structure is done by perturbation theory. The expressions of the first and second order propagation constants, static field distributions, coupling coefficient and propagation characteristic are presented. The feasibility and accuracy of the model are demonstrated by comparing the calculation results with finite difference solving semi-vector wave equation and beam propagation method. A coarse wavelength demultiplexer based on spatial four-waveguide system, including three common wavelengths 1.31 μm, 1.49 μm, and 1.55 μm of the triplex transistor in today's fiber-to-home, is designed with the insertion loss 0.68 dB. Further optimization of the separate outputs and waveguide structure can obtain higher qualities.