An investigation on bioaerosol in a wastewater treatment plant (WWTP) located in Xi'an, China, was conducted to understand the characteristics of bioaerosol released from wastewater and sludge treatment facilities because the bioaerosols may pose a threat to human health. Using the Andersen impactor sampler collection and colony-counting method, bioaerosol concentrations and size distributions were detected. The risk quotient method was used to evaluate the health risks associated with inhalation of bioaerosol for WWTP staff, based on the average daily dose rates of exposure. The health risk in relation to Legionella pneumophila was quantitatively calculated using quantitative microbial risk assessment (QMRA), based on the assumption of the percentage. The maximum concentration of airborne bacteria (3,767 ± 280 colony forming units (CFU)/m3) and fungi (8,775 ± 406 CFU/m3) occurred from the aerated grit chamber and sludge thickening house, respectively, which all exceeded 500 CFU/m3 as the acceptable guideline proposed by the American Conference of Governmental Industrial Hygienists. The particle size of airborne bacteria was mainly distributed in the first three stages (>3.3 µm), while that of airborne fungi was from the second to the fourth stage (2.1-7.0 µm). The hazard index exposure to bioaerosol for adult males and females by inhalation were higher than 1. The proportion of L. pneumophila should be strictly controlled below 10-8, based on the QMRA approach.