X-crossing vertical coupler filters may be used as optical add/drop multiplexers (OADM’s) in wavelength-division multiplexing (WDM) systems to select and route different channels. The basic requirement is to effectively suppress the side-lobes and enhance wavelength selectivity, which is a complex function of structure geometry and material dispersion. To understand the mechanisms and optimize X-crossed OADM, we have studied the relationship between the performance of the coupling structure and some of their geometrical parameters such as the width of drop channel, the cladding layer thickness and the angle between two vertical waveguides. In our designed structure, the cladding layer is undoped InP and the input/through channel and drop/add channel are both in the guiding layer of InGaAsP material. The field-coupling phenomena of the coupling in the layered structures are simulated by three-dimension beam propagation method (3-D BPM). Four distinct parameters have been identified to characterize the device performance, namely the peak wavelength, 3dB bandwidth of the main peak (drop channel), side-lobe level, and crosstalk, which is defined as the relation between drop efficiency and the throughput attenuation at resonance. The results show that to obtain a decreasing trend of the peak wavelength, increasing the width of drop channel is the only efficient way. Additionally, we observe the 3dB bandwidth shrinks because of the above variation. Increasing the angle contributes more to the side-lobe level suppression, whereas the cladding layer thickness improves the crosstalk, maximizing it up to 27.22dB. The simulation gives optimization ranges for each geometrical parameter, outside which the intensity profile tends to split into two. This work provides some useful optimization results for designing the X-crossing vertical coupler based OADM.