Aerosol diluter is an instrument for diluting high-concentration aerosols into low-concentration aerosols to meet applicable range of aerosol testing instruments such as particle size spectrometers. The instrument collects a portion of aerosol through a fine capillary with constant flow rate, the rest of aerosols are filtered out by HEPA filter via bypass, then two streams in capillary and bypass are evenly mixed to dilute particle concentration in capillary. Particle dilution ratio can be fixed by maintaining a certain ratio of capillary to bypass. Dilution ratios are up to 100 times.