Abstract: Based on the repetitive adverb fu (复), in about Han dynasty, modal adverb fu come into being. Researching its formation, there are two significant factors. One is the combination of fu and interrogative pronoun shui (who) or he (what), and the other is the analogizing effect of the modal adverb you (again). As a new modal adverb, fu is not only used in interrogative sentences to express cross examination or question, but also used in negative sentences or affirmative sentences to express emphasis. However, being deeply affected by its original verb meaning, modal adverb fu is not highly grammaticalized. Meanwhile, the suffix fu which enjoys the same form gives it the most serious attack. On the other hand, it is also squeezed by the other related modal adverbs, such as you (again) and hai (moreover). As a result, in the Tang and Song Dynasties, the use of fu is on the decline. At last, in about Yuan dynasty, modal adverb fu begins to disappear.