Abstract: Multi span plastic greenhous is expedient and safe to install, economical to invest. It is popular in China. This paper analyzed existing typical structure of multi span plastic greenhouse(GLP-728 in Hang zhou area) by means of structural mechanics, and worked out a kind of safe and economical structure by use of optimum structural design theory and software of finite element analysis. After the structure was optimized, the intensity utilization coefficient of horizontally side post, arch steel tube and mid post increased 18%, 3% and 25% respectively, the vertically side post which was utilized insecurely and insatiably utilized securely, stably, and reasonably, and steel of main body was saved about 3.7%.