A two-stage filtration testing system, consisting of a pre-stage filter and a main-stage filter was successfully developed, which proves capable of being used to investigate the loading characteristics of a two-stage filtration system. A G4 filter media was selected as pre-stage media sample while an Ell filter media was tested as mainstage sample. Bi-modal aerosols consisting of both super-micrometer A2 coarse dusts and sub-micrometer KCl fine particles with varied mixing ratios were employed to challenge the two-stage filtration system to investigate the influence of aerosol size distributions. Two sets of self-defined performance-based indexing parameters were introduced to quantify the effectiveness of the pre-stage filter and simplify the comparison among various test conditions. It was found installing a pre-filter in front of a main-filter can improve the performances of the entire filtration system, by extending the service lifetime of the main-stage filter and increasing the dust holding capacity of the whole filtration system. Moreover, the effectiveness of pre-stage filter was found to be strongly influenced by the particles size distribution: the protection by a pre-stage filter is more pronounced for large particles and tends to fade out with increasing fraction of fine particles. The specific cake resistance K-2 of the main-stage filter was found to increase with increasing fraction of fine particles, and was higher at the presence of a pre-stage filter than the corresponding reference case without a pre-filter.