Objective: To understand the exposure to second hand smoking (SHS) and associated factors among middle school students in Beijing, and to provide data support for tobacco control. Methods: The two stage stratified cluster random sampling method were used to select 10 532 students from 370 classes in 48 junior middle schools, 34 senior high schools and 14 vocational high schools in 16 districts of Beijing. The national unified paper questionnaire was used to collect the information. Results: During the past 7 days, 71.5% (95% CI =70.2%-72.7%) of students reported exposure to SHS. The proportion of exposure was highest (60.3%) in outdoor public places, followed by indoor public places (48.9%), at home (34.1%), and public transport (19.1%). About 31.6% of students reported people smoking in the campus in the past 30 days. Risk factors of SHS exposure included one or more parents was smoker( OR =2.62), friends who smoked( OR =2.13), received education on tobacco hazards in school( OR = 0.74 ), and senior high school( OR =0.68-0.73)( P <0.05). Conclusion High exposure to second hand smoking among middle school students in Beijing is common. Implementation of the regulations and the publicity of tobacco hazards and tobacco control in schools should be strengthened. Smoke free household should be advocated, and middle school students, especially junior middle school students, should be protected from the harm of SHS.