Abstract: In this study,ultra-pure water,mineral water and tap water were selected as humidifying water sources.One brand household ultrasonic humidifier was adopted to humidify on a closed office.By using intelligent flow PM_2.5 sampler,FA-3 aerosol particle size sampler and wide-range particle size spectrometer (WPS),the indoor aerosol mass concentration,mass concentration spectrum,particle number size distribution were measured and water soluble ions in aerosol were measured by chromatograph analysis.The results show that indoor PM_2.5 mass concentrations have obvious growth and dominant of water soluble ion after humidification by different types of water.The net increase by humidification of ultrapure water,mineral water and tap water in indoor PM_2.5 were 16.9,46.0 and 428.2μg·m^-3 respectively.The humidification is mainly to increase the mass concentration and ion concentration of the fine particle size range below 2.1μm.The increase of major ions (SO₄^2-,NO₃^-,Cl^-,Na⁺,Ca²⁺,K⁺) were directly related to the most significant ions in the types of water.WPS measurements show that indoor aerosol number concentration increases significantly with a peak at about 20 nm,after humidify by ultrapure water and mineral water.While humidification by tap water,number concentration increases with a peak at about 30 nm.All types of water humidification lead to the number concentration increment of more than one order of magnitude.The increase of mass concentration by WPS peaks in size range of 500-600 nm after humidification,which is in accord with the measurement by aerosol particle size sampler that the peak increment at 0.43-0.65μm.When the ultrasonic humidifier was used,the pure water is recommended for health safety.