Filter paper can be dramatically improved by the slip-flow effect and the huge specific surface area of nanofibers. Nanofibers can provide significant improvement in filtration efficiency with a relatively small reduction in permeability. In this study, nanofiber was prepared by fibrillation method using para-aramid fiber. A lab method was developed to laminate the nanofiber layer on a paper substrate by a wet-laid method. The filtration performance of dual-layer filter paper with fibrillated nanofibers was evaluated, and a theoretical model was developed to study the impact of nanofiber diameter on the filtration properties. It is found that in the fibrillation process, the number of trunk fibers decreased and nanofibers increased. With smaller fiber diameter, the pressure drop and filtration efficiency of dual-layer filter paper increased. Based on fibrous structure analysis, the dual-layer filter paper was able to perform well in self-cleaning filtration. The modeling results for pressure drop and filtration efficiency were close to the tested results, which provide a tool to understand the relationship between nanofiber diameter and filtration performance.