Abstract: In order to simply and rapidly measure catalase (CAT) activity in plant and explore the correlation between CAT activity and time variation, ultraviolet spectrophotometry was applied to compare the time course of the decomposition of H2O2 catalyzed by CAT in four different reaction systems. The results showed that the decomposed reaction of H2O2 catalyzed by CAT accorded with the first order reaction in a certain substrate concentration. So the constant value of rate K could be represented the activity of CAT. Compared with other reaction solutions, 0.03% H2O2 had such advantages as simple preparation, low cost and could get the best time course in the measurement, so it was the optimum reaction solution of CAT activity measurement.