When nanofibers could be made last century, its filtration potential was expected in a lot of professional fields, such as building industry, medical industry and Environmental protection industry. In the present study, in order to prove and explain its performances, nanofibrous membrane with given thickness was electrospun with 8% PVA solution on conventional cotton scaffold. Then by means of special air filtration test system, the filtration performances between PVA nanofibrous membrane and conventional fibrous filters were confirmed and compared. Besides, the filtration efficiencies of particles distribution 0.3-5um were attained, and at the same time, another important parameter, pressure-drops, could be observed, too. Through data analysis, the differences of filtration performances were verified. Despite the fact that nanofibrous membrane had higher pressure-drops, its filtration efficiency could reach startlingly 99% for some given particles. In succession, SEM was used to observe the particles distribution in filter, and the different filtration mechanisms were analyzed and discussed with SEMS images. Furthermore, we found that the direct inception and the inertial impact improved nanofiber filtration efficiency, while too small diameter of nanofiber induced higher pressure-drops. In general, nanofibers have huge value in air filtration application.