Little is known about the reasons for the high dropout rate of students from poor Southwest China families under various policies support. The dropout rate of 27312 junior high school students was 6.90%, and the dropout rate of grade 7-9 increased with the grade. Principal Component Analysis was conducted on 21 attributes of 299 dropout students. We found that parents with higher educated degree and late marriage, parents working close to home, large family size, family with sick or disabled members, higher grade and non-boarding and unhealthy students increased the dropout rate. The dropout prevention requires reducing the inducers from family, school management and students themselves.