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Flow field structure in the agitated tank has both isolated and chaotic mixing regions.In the chaotic mixing region,the fluid is stretched in Lyapunov exponent with complicated trajectory,which contributes to good mixing performance.Largest Lyapunov exponent (λmax) is an important parameter to characterize the chaotic mixing region.Pulse pressure signal on the inner wall of the agitated tank was collected by pressure sensor and the software LabView.λmax was calculated by software Matlab based on pulse pressure signal data.Results indicated that λmax varied with the rotation speed (N) and chaotic mixing was obvious at certain range of rotation speed.λmax is a deflexed parabola function of N.Surface tension also played an important role in flow filed and changed its largest Lyapunov exponent.Steady and symmetrical flow field tended to occur at high rotation speed,which decreased λmax and mixing performance.