The sulfurization of several heteropoly acids (HPA), which have been applied as catalysts frequently in recent years, have been studied by IR spectra and temperature programmed sulphurization technology. The result showed that sulphurization of PMo12 is the easiest one among the three kinds of HPA (PMo12, PW12, SiW12). In addition, the trend that the two kinds of bridged oxygen in HPA were substituted by sulfur atoms prior to terminal oxygen, is PMo12 >PW12 >SiW12. In the initial stage of sulphurization, some of oxygen atoms in HPA are substituted by sulfur atoms reversibly and the Keggin structure has been preserved during this process. For PW12 and SiW12, the Keggin structure can be preserved until the sulphurization temperature has been raised to 673K, and for the PMo12, this temperature is near room temperature. When the temperature exceed those mentioned above in the sulphurization process, the exchange between sulfur and oxygen become irreversible, and deep sulphurization of HPA may be accompanied with disintegration of the Keggin anion skeleton.