In this study, desilication treatment using alkali etching was applied to ZSM5 molecular sieves with four different silicon-aluminum ratios to investigate the difference of their ammonia nitrogen adsorption. Their changes in crystal structure, surface morphology, internal element composition and connection mode after desilication were examined with scanning electron microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, and Fourier transform infrared spectroscopy. The mechanism of the desilication effect on improving the adsorption of ammonia nitrogen by molecular sieves was then analyzed. The results indicated that after desilication treatment, the mesoporous specific surface area and specific pore volume of the molecular increased, and the pore size distribution was broaded, the structure of the molecular sieve remained intact and the grain morphology became clear and regular, associated with enhanced ion exchange capacity and Si/AlO₄ structure. The molecular sieve desilication by alkali etching had multiple effects such as desilication of skeleton, the aluminum supplementation of skeleton, and the tunnel dredging. These led to decreased silicon-aluminum ratio and increased number of active sites in the molecular sieve, meanwhile, decreased the diffusion resistance of ammonia ion, which then resulted in improved ammonia nitrogen adsorption performance. After desilication, the ammonia nitrogen adsorption by molecular sieve was greatly enhanced, while more significant improvement was observed for molecular sieve with originally higher silicon-aluminum ratio. while its equilibrium adsorption capacity of ammonia and nitrogen increased from 5.81mg/g to 10.44mg/g, with an increase of 79.7% being reached. In summary, the desilication treatment could provide an effective technical alternative to use molecular sieve for the deep denitrification of wastewater treatment plant effluent.