► Evaluate the impacts of low intensity ultrasound on catalytic activity of cellulase. ► The mechanisms of ultrasonic impact on cellulase were investigated. ► The free and immobilized cellulase activity were increased by 18.17% and 24.67%. This research was to explore the mechanism of ultrasonic impact on free cellulase activity and immobilize cellulase activities. The highest free cellulase activity was achieved when the sample was treated with low intensity ultrasound at 15W, 24kHz for 10min, under which the enzyme activity was increased by 18.17% over the control. Fluorescence and CD spectra revealed that the ultrasonic treatment had increased the number of tryptophan on cellulase surface slightly, with the deformation of certain number of α-helix structure and increase of random coil content in cellulase protein. The highest immobilized cellulase activity was achieved when the sample was treated with low intensity ultrasound at 60W, 24kHz for 10min, under which the enzyme activity was increased by 24.67% over the control. Scanning electron microscopy revealed that the ultrasonic treatment had increased the surface area of immobilized cellulase.