This study focused on the grammaticalization of tou(透)， using the theory of grammaticalization analysed the relationships among the meanings of tou., and discussed the conditions and mechanisms during the grammaticalization process of tou. According to the results of this study, the ethmological meanings of tou-- guo and tiao, were great influnce on the meaning development of tou. The common semantic features between guo and tiao are [out of the border]and [transendence]. Beside that, reanalysis was also played an important role on the grammaticalization of tou. Through reanalysis, the verb tou got the function as a complement. The grammaticalization follows the principle of continuity, which has the advantages of semantic and syntactic conditions. We found out that metaphor, reanalysis and analogy played important parts together during the grammaticalization process of tou.