Observations of atmospheric air ions at most geographic areas have been undertaken, but there are still very few observations at middle-latitude semi-arid areas throughout the world. To quantify characteristics of small air ions in semi-arid areas of China, measurements of small air ions and particulate matter (PM10) were conducted at 16 different sites in Xi'an (34°26′N, 108°94'E), China, from August 2009 to July 2010 along with meteorological parameters. The influences of meteorological parameters and PM10 concentration on small air ions as well as their influence mechanisms were discussed in detail. The results showed that both spatial and temporal distributions of small ions were uneven in Xi’an city. Spatially, the concentration of small ions increased gradually from city center to the suburbs. The mean annual concentration of small negative ions was in the range of 200–400 cm–3 with the mean value of 261 ± 39 cm–3 at urban areas, while more than 500 cm–3 at suburban areas. Significant seasonal variations of small ions were observed with highest level in summer, followed by spring and autumn, and lowest in winter at various types of sites. The positive correlations were found between small ions concentration and temperature as well as relative humidity, while negative correlation between the small ions concentration and PM10 concentration. Air ions concentration was also applied to the assessment of air quality in Xi’an. The assessment results indicated that the air quality was better at suburban areas than at urban areas, suggesting that the unipolarity model is reasonable to indicate air cleanness.