Based on the general model of two parallel long-period grating coupler, the device stability was investigated by studying the grating longitudinal offset. Influences of modulation depth, surrounding refractive index and fiber cladding thickness were analyzed and the conditions for achieving 100% add_drop filtering was obtained. The measured experimental results demonstrated that efficient add_drop channel filter could be achieved even if the claddings were not reduced, which provided a method to simplify device fabrication in the further.