Objective To explore the learning outcome of “flipped classroom” method on the theoretical teaching of surgery for medical undergraduates.Methods Randomized controlled trial was carried out.In 2018,92 medical undergraduates from the 2015 five-year major were randomly allocated into two groups:control group and experimental group,with 46 students in each group.The experimental group received “flipped classroom” teaching method,while the control group was taught by traditional teaching method.The teaching outcomes were evaluated in three ways:students' behaviors in the classes;results in two exams with a three-month interval;questionnaire for students about their learning outcome.Results Compared with control group,experimental group showed more question asking,significantly higher scores in the two exams[(83.18±3.71)vs(74.64±3.42),(77.91±3.91)vs(63.82±3.28)],with statistical significance (all P<0.01),as well as better comment on their own learning outcome in the questionnaire,especially in regard to learning interest,ability enhancement and vision expansion[(8.86± 0.54)vs(7.57± 0.46),(9.34± 0.47)vs(7.19± 0.36),(9.20± 0.52)vs(7.33± 0.41)],with statistical significance(all P<0.05).Conclusions Reasonable application of “flipped classroom” method in the theoretical teaching of surgery knowledge could effectively improve the learning outcomes by strengthening students' learning interest,the accuracy and comprehensiveness of their knowledge understanding,their long-term memory as well as comprehensive abilities.