Abstract: Field research was conducted on 7 host tree species and lac insect population dynamic of lac insect during estival generation. The results are as follow: Different host tree has different growing speed, the increment of the area of branch without lac insect is up to 30%～50% among different host trees at the end of lac insect life history, otherwise, the increment of the area of branch with lac insect is up to 15%～40%; the branch has different growing speed in different orientation, the longitude growth is 5 times faster than transverse growth; the host trees of lac insect grow slowly at the beginning of lac insect life history, but grow fast during lac insect adult period. Differently aged branches of the same host tree have different growing speed. Younger branches grow faster than order ones. Host tree of lac insect grows slower during winter than that during estival. Lac insect has higher mortality in larvae stage, and the total mortality of lac insect is more than 90%. Former formula for lac insect mortality M=(d-d')/d, ignores that host tree growth has influence on the statistical result of lac insect population. According to the truth that host tree of lac insect grows continuously during the whole life cycle of lac insect, an improved new formula to count mortality of lac insect M=(d-Cd')/d is proposed.