The [Bmim]OAc，[Bmim]2CO3 and [Bmim]OH ionic liquid were synthesized and their structure were confirmed by IR、1HNMR and elemental analysis. First, Chitosan was alkalized by ionic liquid. Then the alkalized chitosan reacted with bromododecane. And thus N-dodecyl chitosan with high degree of substitution (DS) was produced. The structure of alkylation product was characterized by IR、1HNMR and XRD. It studied the effects of time, temperature and the ratio of reactant on the DS of N-dodecyl chitosan. The suitable reaction conditions were found as follows：n([Bmim]OH):n(chitosan)=3:1，the chitosan was alkalized for 1h with alkalization temperature 45℃，n(bromododecane): n(alkalized chitosan)=2:1，the alkalized chitosan reacted for 3h with alkylation reaction temperature 80℃. Under above conditions，the DS of N-dodecyl chitosan can be over 81％. The reusability of the ionic liquid was also investigated，and it was found that the DS still could be kept above 80％ after the [Bmim]OH ionic liquid was reused for 3 times without any retreatment.