The highly pure nanocrystalline Mg,Al-hydrotalcite with Si4+ doping was synthesized by the one-step liquid reaction method at atmospheric pressure. The thermal properties of the Mg,Al-hydrotalcite with / without Si4+ doping were investigated; and two thermal decomposition process stages were recognized. The effect of the Si4+ doping on the thermal properties of the Mg,Al-hydrotalcite samples is discussed according to the results of XRD, TEM, IR and DSC-TG. Moreover, the thermal mechanism functions of the Mg,Al-hydrotalcite with / Without Si4+ doping are studied in detail after calculation and comparison.