The l-lysine functionalized polyacrylonitrile fiber (PAN(L)F) was prepared by grafting the l-lysine into a commercially available polyacrylonitrile fiber and showed highly catalytic activity for Knoevenagel condensation reaction. With low temperature (45 A degrees C) and short reaction time (1 h), the fiber catalyst was well applicable to Knoevenagel condensation of a wide range of aldehydes and the yields could reach up to 99%. Interestingly, only in water could the reaction take place smoothly (with a yield of 88%) and a polar micro-environment promoted reaction process had been proposed to explain this phenomenon. Besides, the fiber catalyst has advantages of easy preparation, high functional degree, strong mechanical strength and thermal stability, etc. And it can be reused at least 5 times without further treatment and performed well in scaled-up experiment (amplified 50 times) and flow chemistry experiment (no loss of catalytic activity after 48 h), which indicates its potential application in industry application. [GRAPHICS]