BACKGROUND: The China Mental Health Survey was set up in 2012 to do a nationally representative survey with consistent methodology to investigate the prevalence of mental disorders and service use, and to analyse their social and psychological risk factors or correlates in China. This paper reports the prevalence findings. METHODS: We did a cross-sectional epidemiological survey of the prevalence of mental disorders (mood disorders, anxiety disorders, alcohol-use and drug-use disorders, schizophrenia and other psychotic disorders, eating disorder, impulse-control disorder, and dementia) in a multistage clustered-area probability sample of adults from 157 nationwide representative population-based disease surveillance points in 31 provinces across China. Face-to-face interviews were done with a two-stage design by trained lay interviewers and psychiatrists with the Composite International Diagnostic Interview, the Structured Clinical Interview for DSM-IV Axis I disorders, the Community Screening Instrument for Dementia from the 10/66 dementia diagnostic package, and the Geriatric Mental State Examination. Data-quality control procedures included logic check by computers, sequential recording check, and phone-call check by the quality controllers, and reinterview check by the psychiatrists. Data were weighted to adjust for differential probabilities of selection and differential response as well as to post-stratify the sample to match the population distribution. FINDINGS: 32 552 respondents completed the survey between July 22, 2013, and March 5, 2015. The weighted prevalence of any disorder (excluding dementia) was 9·3% (95% CI 5·4-13·3) during the 12 months before the interview and 16·6% (13·0-20·2) during the participants' entire lifetime before the interview. Anxiety disorders were the most common class of disorders both in the 12 months before the interview (weighted prevalence 5·0%, 4·2-5·8) and in lifetime (7·6%, 6·3-8·8). The weighted prevalence of dementia in people aged 65 years or older was 5·6% (3·5-7·6). INTERPRETATION: The prevalence of most mental disorders in China in 2013 is higher than in 1982 (point prevalence 1·1% and lifetime prevalence 1·3%), 1993 (point prevalence 1·1% and lifetime prevalence 1·4%), and 2002 (12-month prevalence 7·0% and lifetime prevalence 13·2%), but lower than in 2009 (1-month prevalence 17·5%). The evidence from this survey poses serious challenges related to the high burdens of disease identified, but also offers valuable opportunities for policy makers and health-care professionals to explore and address the factors that affect mental health in China. FUNDING: National Health Commission of Health (Ministry of Health) and Ministry of Science and Technology of China.