Phosphorus-modified Al2O3 supports containing different P content were prepared to control the acidity of Ni-impregnated catalysts for application in the dry reforming of methane (DRM). Results from the H-2-temperature-programmed reduction showed that an increase in P content promoted the formation of an AlPO4 phase, and resulted in an increase in Ni particle size. Although the initial conversion of CH4 and CO2 decreased with increasing P content, the addition of P influenced on the acidic properties of the support and the catalyst. Indeed, a decrease in catalytic deactivation correlated with the decreasing acidity of the P-modified supports and the Ni/P-Al2O3 catalysts, and the acidic properties were determined by the behaviors of the temperature-programmed desorption of iso-propanol (IPA) and the dehydration of IPA. We found that the Ni/Al2O3 catalyst modified by 2 wt.% P exhibited the weakest acidity whilst demonstrating excellent coke resistance and stability over 100 h in the DRM reaction.