Abstract: Although chitosan is a biocompatible，biodegradable and nontoxic polymer，its application suffers severe limitations because it is insoluble in neutral or alkaline media. This paper reports the synthese of Schiff base of chitosan by the reaction of chitosan with citral assisted with high-intensity ultrasound. Effect of the ratio of chitosan to citral，reaction time and temperature on the yield and the degree of substitution has been investigated. The optimal conditions were found to be with reaction temperature of 40－50 ℃，the ratio of chitosan to citral of 1∶6 and 10 h of reaction time. The maximum yield was achieved to 86% and the degree of substitution was 0.82 under the optimized reaction conditions. The structure of the Schiff base was characterized by FTIR and XRD spectroscopy，which confirmed that the amino groups in chitosan have been reacted with citral to form the Schiff base. The antibacterial activities of the Schiff base were investigated by assessing the inhibitory rates of Escherichia coil，Staphylococcus aureus and Aspergillus niger. The MICs of Schiff base against E. coli，S. aureus and A. niger were found as 1g/L，1g/L and 5g/L，respectively. The antibacterial activity of the Schiff base increases with the increase in bacterial concentration and is stronger than that of chitosan.