On the basis of requirements for producing high quality tea, the seasonal variation of mechanical compositions of machine-plucked tea shoots under three plucking period was investigated when the following three criterions for tea shoots was pre-set up in one bud with one leaf and one bud with two leaves, and two-leaf banjhi. Results showed that high quality tea could be produced when the tea shoots were plucked in the period that 75.0%~84.9% shoots were met the one bud and tow leaves stage, and therefore this period can be defined as the optimum machine-plucking time. It was also showed that if machine-plucking is performed based on the above-defined criterion, the interval of machine-plucking in spring would be around 20 days, if an optimum period of machine-plucking is defined on the basis of the formation of a fish-leaf, it could be 15 days after the formation of fish-leaf in spring and 12 days in autumn. The average maturing time of a tea-leaf is 5.4 days in spring and 4.2 days in autumn in Hangzhou. Thus, a suitable regulation of machine-plucking period, will obtain the required plucked tea shoots for the manufacture of high quality tea.