The effects of Na2SnO3 and CuSnO3 on the thermal decomposition and fire performance of wood were characterized by thermogravimetric (TG) analysis, differential thermogravimetry (DTG), and thermogravimetry–mass spectrometry (TG–MS) analysis. The addition of these chemicals caused a decrease in the decomposition temperature, a reduction in weight loss, and an increase in the amount of char produced. The results showed that ion current intensity and ion peak area of the typical representatives of incombustible ion such as m/z = 18 and 44 MS signals were increased by the flame retardants but the inflammable ion such as m/z =12, 26, 28 etc. MS signal was decreased at the meantime. What’s more, the maximum ion current intensity and ion peak area of m/z = 64 signals were increased obviously, which mean that the flame retardants can significantly enhances the dehydration and inhibits the deploymerisation of wood.