Activity ratio K a (a ratio between the amount of soluble SiO 2 and soluble Al 2O 3 in saturated CaO solution and total amount of SiO 2 and Al 2O 3 of that material) of pozzolanic material was determined by method of boiling circumfluence. It was shown by testing that there is a preferable interrelation between K a and the standard mortar strength ratio. For the same pozzolanic material, the varying trend of K a is similar to that of strength ratio of mortar as fineness was varied. When the SO 3 content in FBC (fluidized bed combustion) ash was varied, influence of SO 3 on K a of that ash was consistent with that on strength ratio of mortar. Therefore, influence of the other factors on the activity of pozzolanic materials can be explained by K a. It was shown by closed loop test and cross ranged test of testing parameters that determination method on activity of pozzolanic materials is reliable and rapid. The whole test needs only several hours.