The purpose of this study was to investigate the factors affecting somatic cell counts (SCC) and milk composition of Chinese Holstein cows in Ningxia. The correlation of 198 855 measurements of the dairy herd improvement (DHI) data of 19 020 Chinese Holstein dairy cows (from 2009 to 2018) from 7 different dairy farms was analyzed using Minitab software. The results showed that 1) SCC was significantly correlated with farm, year and parity (P < 0.05); 2) SCC fluctuated with season and calving month, and decreased first and then increased with increasing lactation length; and 3) there were significant correlations between SCC and milk yield, milk fat percentage, milk protein percentage, milk sugar percentage and dry matter percentage (P < 0.05). Moreover, SSC, milk yield and the major milk components were influenced by environmental factors (farm, year and season) and cow’s own factors (parity, calving season and lactation period). In summary, fine management is an effective avenue to improve milk production and quality.