<P><B>Abstract</B></P> <P>Effects of nonionic surfactants on enzymatic hydrolysis of Avicel at different agitation rates and solid loadings and the mechanism were studied. Nonionic surfactants couldn’t improve the enzymatic hydrolysis efficiency at 0 and 100rpm but could enhance the enzymatic hydrolysis significantly at high agitation rate (200 and 250rpm). Cellulase was easily deactivated at high agitation rate and the addition of nonionic surfactants can protect against the shear-induced deactivation, especially when the cellulase concentration was low. When 25mg protein/L of cellulase solution was incubated at 200rpm for 72h, the enzyme activity increased from 36.0% to 89.5% by adding PEG4600. Moreover nonionic surfactants can compete with enzyme in air-liquid interface and reduce the amount of enzyme exposed in the air-liquid interface. The mechanism was proposed that nonionic surfactants could enhance the enzymatic hydrolysis of Avicel by reducing the cellulase deactivation caused by shear force and air-liquid interface.</P> <P><B>Highlights</B></P> <P> <UL> <LI> High speed shearing would lead to cellulase deactivation. </LI> <LI> Nonionic surfactants could reduce shear deactivation of cellulase. </LI> <LI> The enhancement of nonionic surfactants is notable at high agitation rate. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>