Spatial X-crossing waveguides consist of two vertically stacked waveguides crossed in an X type shape. These kinds of devices can be served as vertical couplers, vertical coupler filters, vertical optical switches and optical add/drop multiplexers. Based on the perturbation theory, a technique named equivalent field matching method (EFMM) is presented for analyzing the coupling characteristics of spatial X-crossing waveguides. The field profile of the rectangular waveguide is treated as a perturbation to radially symmetric fiber, and more accurate calculation of the coupling lengths of the spatial X-crossing waveguides is presented with the effect of corner regions taken into account. And the three-dimensional full-vector beam propagation method (3D-BPM) is used to verify these analytical results. Comparison of the coupling lengths obtained by the 3D-BPM and EFMM shows that the maximum relative error is 1.2%, and the average relative error is 0.9%. Results show that this proposed method is more convenient compared with other numerical methods. The equivalent field matching method is extremely useful in the accurate design of three-dimensional integrated optical devices based on spatial X-crossing waveguides.