A highly selective means of producing propylene via the ethylene-to-propylene (ETP) reaction is required to manage the supply-demand balance of light olefins. To improve the catalytic activity of the ETP reaction, a phosphorus-modified SSZ-13 zeolite (P-SSZ-13) was prepared by introducing phosphorus to the SSZ-13 zeolite via a simple impregnation method. The P-SSZ-13 catalyst exhibited increased propylene selectivity and yield compared to the pristine catalyst. According to solid-state NMR spectroscopy and STEM-EDS observation, the phosphorus species were mainly located on the outer part of the crystal as a form of polyphosphate. To reveal the effect of the phosphorus loading on the enhancement of ETP reactivity, a series of surface-modified SSZ-13 catalysts were prepared and investigated. It was found that the polyphosphate species in the P-SSZ-13 catalyst not only weakened the acidity of the zeolite by bonding with the framework aluminum species but also reduced the diffusion out of the relatively large-sized hydrocarbon species by partially blocking the pore mouths. The change in the diffusivity of the hydrocarbons over the SSZ-13 catalysts also affected the ETP reactivity. The relationship between the amount of phosphorus and the ETP reactivity of the P-SSZ-13 catalysts could also be clearly demonstrated in terms of the change in both acidity and intracrystalline diffusivity. Moreover, 13C MAS NMR measurement with isotopic switch experiment confirmed the alkyl naphthalene-based hydrocarbon pool mechanism over the P-SSZ-13 for the ETP reaction.