Abstract: According to the results of observation on thin arch dam of Lishimen, the changes of dam body stress during the cnstruction and operation are analyzed in this paper and got the conclusion that the dam body stress under construction is bigger than that in operation, cracks occurred at construction period and itiatial impound, observation stress distribution differs from design calcu-lation, maximum stress is at downstream of central arch crown, tempreture load is main load, of which the tempreture gradient at upstream and downstream and partial linear tempreture difference have a great influence on thin arch stress, the maximum thrust at arch foundation usually works in the summer and so on. The reasons of cracks downstream has been analyzed as well.