BACKGROUND: To investigate the prevalence of pterygium and associated factors in Han and Mongolian adults at four survey sites in Inner Mongolia, China. METHODS: We conducted a population-based, cross-sectional study as part of the China National Health Survey (CNHS). By means of a stratified sampling method, we finally included 2651 participants of 30 years of age or older from a total of 3468 eligible residents. Factors associated with pterygium were analysed by a univariate analysis and logistic regression models. RESULTS: The study population included 1910 Han and 741 Mongolian adults. The mean age ± standard deviation of the study cohort was 48.93 ± 11.06 years. The overall prevalence of pterygium was 6.4% (n = 169); 1.4% (n = 38) of the cases were bilateral and 4.8% (n = 128) were unilateral. The most common grade of pterygium was Grade 2. Based on the results of the univariate analysis, eleven factors were included in a multivariate analysis. The results indicated that age (P < 0.001), outdoor occupation (P = 0.026), and time spent in rural areas (P < 0.001) were significantly associated with pterygium. Sex and ethnicity were not identified as risk factors. CONCLUSIONS: Our results indicated that outdoor occupation, old age and more time spent in rural areas were risk factors for pterygium in Inner Mongolia. At the same time, town as a survey site (Hohhot and Tsining District) was a protective factor for pterygium. Ethnicity, gender, smoking, diabetes and high blood pressure are not associated with pterygium.